I won't deny that I'm guessing at some details of security systems and the like. While I do carry out some research for stories, I draw the line at going up to gallery security guards and asking 'Hello! If I wanted to steal a painting, what would catch me?'
So, there might be some factual errors (and possible typos, I've tried to spell-check but I could have missed some). Still, I hope you enjoy.

They knew the thief had influenced the CCTV footage, somehow; they'd blacked out the camera in the painting's room. Still, there was a huge gap between cutting off a camera temporarily, and reworking the footage to hide the path of a thief, and doing so, so successfully that even the time given on each screen had been changed.

"Was there anyone watching the footage from these cameras?" John said, sitting beside DI Lestrade.

They were again in the gallery, watching past footage on the monitors. While they two watched the screens, a small team of computer experts were designing a way to track the looped sections of video.

The team used a map to determine a possible point for the thief to take to enter the view of the camera that watched Lauren. From there, they used a rough guess of the time that the thief would have been there, and searched the rest of that camera's footage for an identical segment.

So far they'd tracked the thief's path through three cameras. It seemed that they had in fact been able to just walk through the front door: though they hadn't determined the route fully. At present, they had four possibilities to look through.

"Sure, Hans Sharma, 32. He's being interviewed right now," Lestrade lay back, watching the screens as they played back the looped sections. "We can tell that he has to be aware of what's going on; he would have seen the CCTV as it came through live. Still, he's not saying anything."

There was no way to tell how many of the guards were involved in it as well; perhaps the thief had walked past in full sight of them, or perhaps something else had happened in the unknown sections. They couldn't tell.

"Is there any chance he could have just missed them?" John said, eventually

"None, if he was doing his job," Lestrade seemed annoyed. "Hans was watching the camera that kept an eye on Lauren. That's the only one we're certain the thief walked through: some of the specialists out there," he gestured towards the at-work team of computer experts, "Reckon there's more than one trail. Then we can't even tell what route the thief took."

John frowned, thoughtful. If multiple cameras featured looped sections of video, it could have meant that there were multiple people heading in: but that seemed excessive. It did make a brilliant distraction; the route alone might not hold too many clues, but it may still hold some. It would take the police much longer to search several possible routes.

And, of course, it was a display of power. Editing a few CCTV cameras wasn't anything special; or rather, it was, but not compared to having seeming free reign.

"There was no one else with Hans?" John spoke, wishing there was a way to come up with more information. They didn't even have any decent suspects any more: Hans Sharma was the only one, but couldn't have committed the theft, and didn't even have to necessarily know who did.

Anonymous commands and blackmail were nothing new; if someone had influenced Hans in that way, it would explain his reluctance to come forward with anything. Then again, so would plain and simple guilt.

"Meant to be," Lestrade replied; "His partner was out to the toilet at the time of the theft."

"Convenient," John said, frowning. That was a definite mark against Hans' innocence: so well timed. "So if we assume Hans Sharma is the only one of the CCTV people in on the crime, was he watching all the screens, or just some?"

"Just some," Lestrade grunted. "With the size of this place, he couldn't watch them all. There were others for other sections."

"How much of the tampered footage comes from his screens? Enough for a route from an entrance to the painting?"

Lestrade's eyes widened as he realized what that would mean. It was a way to tell roughly how many people would have had to be working on the theft: if Hans' screens alone couldn't allow the passage a thief, then there would have to be more people involved; and the case expanded so much more.

And Sherlock called it trivial?

"I'll let you know when the team comes up with an answer," the DI shrugged, remembering the limited progress made.

Whoever had edited the CCTV footage was talented. There was no denying that; the team assigned to finding the flaws did little themselves, beyond feeding times to a computer. They just told a program to compare an estimated time with anything else recorded by that camera.

They'd now found, however, that the predicted path they'd begun to unearth was in fact one of many: going from the painting room outwards, one person's curious scanning of an alternate room found a second path and, moving on from that, countless others.

Not every camera had been altered, of course: but most had. It was beginning to appear as though it was more likely, at a random guess, to find a looped piece of footage compared to an unchanged one.

"Call me when you do," John sighed, standing up. "I want to check something."

Watson left Lestrade with the screens, still thinking.

Firstly, the DI was thankful for a security measure implemented by the gallery: in case of just this suspected event, they made sure each one of the surveillance people, such as Hans, didn't know what areas they'd be watching in advance. That way they could pinpoint who, if anyone, had committed a crime.

Second, he wondered: if their mystery thief had created false routes on the cameras, who was to say they had walked in that way at all?

Back to square one.

John stood outside the gallery, looking up to the wall. He could see the hole that the painting must have left through.

At first glance, to him, much of the case seemed to be grounded in delaying tactics. There were so many things to examine, more than there normally seemed to be on such crimes. Then again, maybe he was just used to working with Sherlock.

Whatever the case, it seemed as though the thief was just trying to waste police time. They'd concocted a smokescreen: CCTV editing, traitors among the guards, the hole in the wall…

Even the hole was larger than it needed to be, for the painting. Perhaps they'd intended to encourage John's theory that a child had snuck through, while making it too small for their initial suspicions.

No. Sherlock had called it trivial: even with his mind, it seemed odd that he'd call a mess of loyalties and routes 'trivial'. So there was another answer: John was pleased with himself for that deduction.

He knew one thing for certain. The painting had left this way: Sherlock had said so, and it fitted the facts they had. Someone might be able to sneak past guards unencumbered, but holding a Turner painting? Unlikely.

He'd assume their loyalty, for now. According to Lauren, and the police report, they worked for Mycroft Holmes. They couldn't be leading a double-life with him as their boss.

Which left what?

They couldn't have entered through the gap in the wall. Becky Massey was too big, and she was five: even when he'd suspected Lauren properly, he'd found it odd to involve a child in such a complex theft.

And they couldn't have entered through the door; the CCTV would prevent that.

The solution was obvious; John almost hit himself, beginning to run back into the gallery. They were already in there.

The room where the Turner Painting hung wasn't open to the public; neither were much of the 'staff only' corridors surrounding it. But there had to be a point when the painting's room was accessible, even if at a point when the painting wasn't in there.

So long as the thief knew that the Turner piece would end up there, they could find a hiding place in the room, and wait. John guessed they'd been above the ceiling boards.

And, of course, they would have had to work for the gallery, and be quite important. Otherwise they wouldn't know the location of the painting, nor could they leave false trails in the CCTV. It all fit: unfortunately, so did the other theories.

This one, however, had the distinction of matching Sherlock's brief description: trivial. Plus the detective had been inside the ceiling, and so able to no doubt see evidence of someone else residing there.

"Lestrade," John called the DI, thankful the phone didn't ring too long. "How long have the staff known the Turner painting would be there? And when did it arrive?"

"Huh?" there was a pause as he checked: "A week for the staff. It was put there two days ago."

"And have there been any staff absences in the last week? Illness, unexplained, holidays, anything: anything that happened after they found out about the painting, before the painting and guards arrived, and is still going on?"

A pause as Lestrade sorted out John's semi-babble. Then a few more second ticked by as he checked.

"Two," said the DI: "Joe Fleming, janitor, and Deborah McKenzie, exhibition planner."

Well, John reflected, it was unlikely to be the janitor. That left Deborah. If she planned exhibitions, she could have had a lot of influence in planning how the Turner painting would be displayed.

Of course, it could be yet another distraction: yet it seemed few people would be able to encourage an absence to take place, and fewer would be lucky enough to find one that would last the right amount of time, by the right person, with such a valuable painting present.

"Thanks, Lestrade," he spoke, already re-entering the gallery. "I think I've got an idea of what happened. Not complete yet, but I think it's right, so far."

Two questions were at the forefront of his mind. The first was escape: how could Deborah leave the room once she'd taken the painting? And the second was still on his mind; was it really so easy to push the painting out the wall? Even if the hole could be made silently, the theft had taken place in daylight. Surely there would have been people outside?

It was useless speculating, it seemed. He wished Sherlock was actually helping.

Back in the painting's room, the ceiling had gone. Or rather, fallen.

Upon hearing John's hypothesis, they'd moved each individual board in the ceiling down to the floor, in the same basic shape, creating a more spacious, better lit view of where presumably-Deborah would have lived for several days.

There would be traces, of course: and there were. A few pieces of litter were found; packaging for long-lasting foodstuffs, and water bottles. Certainly enough for someone to live up there, if not comfortably, then adequately.

Efforts soon went into tracking Deborah McKenzie. She'd called in sick, and then hadn't been seen. A small police group sent to her house failed to uncover any trace of her living there.

It didn't explain how she'd escaped, though John hoped they'd find that out when they found her. John was almost disappointed: it was, as Sherlock said, relatively trivial.

Still, he was glad that the case had been solved. Watson returned to Baker Street, fairly happy.

"Solved it!" John called as he entered the apartment, anxious to show Sherlock that he wasn't completely useless. Still, he had enough time to sit down comfortably before the detective entered the room.

"About time," Sherlock didn't seem hugely impressed. "Who do you think it was, then?"

"Deborah McKenzie," John replied quickly; "Lived in the ceiling area for a few days, when there were no guards, and slipped out when it was clear."

The detective suddenly broke into a grin: if it was anyone else, John would have expected a cheer. Yet Holmes didn't respond to his comment, instead pulling his phone out from his pocket, and quickly dialling.

"Yes, Mycroft? He said in the ceiling. I win," his grin barely faded as he hung up.

"What?" John said, flatly, a little taken aback by Sherlock's expression.

"Bet with Mycroft," a shrug; John rolled his eyes.

"So is this what you do, then?" he couldn't quite decide how to react. "Make bets with your brother about whether or not I can solve a case?"

"Not usually," the detective didn't seem to notice John's reaction. "Only when I know I'll win. Oh! And you're wrong, they didn't hide in the ceiling."

Great. If Sherlock was right, as normal…

"Wait," John spoke up suddenly, frowning: "How do you know that? All the evidence is against her; she's been off work for the exact period of time, and she's vanished, and there's no other way to-"

"I counted fifteen possible false trails from the carpet threads and walls in that room," Sherlock interrupted, abruptly silencing John. "Possibly sixteen if you count the scratches behind the painting. The thief was clever: they wouldn't leave rubbish behind, unless it was a trick."

"Hold on, fifteen?" John quickly said, sincerely hoping the detective was exaggerating. If that was how many a cursory glance revealed to the detective, how many more would the police uncover?

"Of course," he shrugged as if it were obvious: "Tear patterns, pulled threads from the carpet, three distinctive, mutually exclusive kinds of pressure marks; four kinds of dirt scattered around, masking whichever was really brought with them. Scratches on the wall made at inconsistent heights; two-"

"Ok, Sherlock, I get it," John exhaled, a little taken aback.

Maybe his definition of 'trivial' was more suspect than he thought. If he found that many false trails from one room, without knowing about the CCTV, then…

More worrying, though, was the criminal that would think of all that.

"You're sure, though?" John spoke again, reaching for his phone. "It wasn't just a bluff, or something? Make you think she was clever, then take the obvious path?"

"Sure? Of course I'm sure," Sherlock muttered; "However many trails are made, it's usually possible to determine which was the real one. Marks of intensity, or of time; if it took longer than they thought… It only takes eyes."

"I have eyes, you know, Sherlock."

"Then use them."

And he was back to insulting. John withheld a sigh, before calling Lestrade. He needed to let him know Sherlock's deductions; at least the detective had said something, even if it was just 'you're wrong'.

Then, wordless, he waited for the end of the day. He didn't plan to return to the gallery tomorrow, unless the police called him, or he came up with some new brainwave.

So far, he was waiting to hear whether a path to the painting's room could have happened on Hans Sharma's screen alone; and to hear if they found Deborah.

She was still John's top suspect; apparently as Sherlock had guessed. She was meant to be ill, and yet had vanished: though, admittedly there were plenty of ways to blackmail or convince someone to pretend to be ill. Still, what other way was there to get into the room?

It didn't seem possible to find a path past all the guards; it seemed unlikely they all looked away right on time, but it seemed just as bizarre to suppose they were all traitors.

Forget who did it: how did they do it?

It was late next day, still without any answer, that John received the phone call. Sighing, he sat back down; still in 221B.

"Sergeant Donovan?" John spoke, a little surprised at the call.

"John Watson," she mimicked him for a moment, before pausing. He could hear the sound of a car in the background: "We're coming over. Think we've solved the case; if you're out investigating, get back to Baker Street."

"Right, I'm there," John frowned, standing up to glance out the window. Why did they want to meet here?

"Good. Is Sherlock there?"

"Can't you hear him?" John moved the phone from his ear, pointing it in the general direction of the shut door to Sherlock's room, and the perpetual violin music coming from it.

"Make sure he doesn't leave."

At that, John hesitated. Knowing what Donovan thought of him, and judging by that edge to her voice… His thoughts were cut off at the sound and sight of a police car parking outside.

"Tell me that's just because you want his opinion," John spoke, knowing it was in vain as he peered down at the car.

"We have evidence: he was behind it," Donovan said, before hanging up. John watched she and Lestrade entered 221B Baker Street. Lestrade looked anything but glad.