The first time John found the key, it was on the counter in the kitchen, halfway between the toaster and the sugar bowl. It was obviously too small to be for a door - some sort of diary or jewelry box, maybe. Sherlock must have put it down to fiddle with one of his experiments (he only rarely actually went in the kitchen to make something as plebeian as tea, so an experiment seemed the much more likely explanation) and then must have forgotten it. John pocketed the key and then forgot it just as thoroughly in the rush to get down to Lloyd's and catch up to Sherlock before he tried to apprehend the adulterous banker and her soon-to-be-ex-boyfriend's new mistress by himself.
Thus it was fairly late in the evening when John found himself curled up in the armchair, trying to word things properly for his blog, and realized Sherlock was storming around the flat in a tightly bundled ball of frustration.
"I can't have lost it. It can't be lost, John, so where is it? It was supposed to be somewhere obvious!"
John sighed and took the bait. "What did you lose, Sherlock?"
"My key! Little silver thing, opens a lock, very necessary for me to find right now and I deleted where I put it and now I can't find it."
Oh. "You left it on the kitchen counter. It's in my pocket."
Sherlock's sudden stillness was startling. He turned only his head, pinning John with that fanatical energy in his gaze. "I need that key."
John knew he probably should have just handed it over, but it was late and he was sore from chasing Sherlock through four blocks' worth of back alleys so the damn detective wouldn't get himself shot before the police could corral the bank-robbing ex-boyfriend and perhaps John wasn't in the most generous of moods. And it was interesting to see Sherlock in this state - usually he was more controlled than this.
"What's it for?"
Sherlock's brows lowered. "Give me the key, John," he demanded.
"Not until you tell me what it's for."
"Why would you care?"
John shrugged. "I'm curious."
Sherlock glared at him for a long minute, but didn't answer. John eventually shrugged again and went back to typing.
"John."
"No."
A long pause. "Fine." And then John heard Sherlock stomp off to his room.
Sherlock was in a snit for the next two days. Usually there were at least three or four days after a case where he was relatively normal (for Sherlock), starting new experiments and poking at his blog and actually being civil to John most of the time, but the key clearly had him worked up. John enjoyed it, in a perverse sort of way - it wasn't often he got to see the detective so riled up about something outside his control, and for once this was something John could out-stubborn him about. If Sherlock didn't want to explain, John didn't particularly see the need to return the key.
It wasn't for a case, or Sherlock would have said so. It didn't appear to be for any experiments, new or ongoing. John rather suspected it was a diary, and the idea of Sherlock keeping a locked diary like a schoolgirl was terribly amusing. Maybe John would demand to read a page or two before he gave the key back . . .
Sherlock searched John's room, of course, but John was prepared for that. He kept the key in his pocket at all times, and he even took it with him into the shower so Sherlock couldn't rifle through his clothes that way. His favorite pajamas had a convenient drawstring on the waistband, which he threaded through the hole in the key, and he was fairly sure he would have awakened if Sherlock had tried to go anywhere near that. It was rather fun to see his flatmate get more and more agitated as time went on - Sherlock would have to apologize and explain eventually -
The apology never came, but the mystery was solved just over two days later. John was sitting at the kitchen table, thoughtfully munching a piece of toast and trying to decide what to do with the two hours he had before he had to be at work, when Sherlock appeared in the doorway. His eyes were bleary and his hair was even more of a mess than usual, and he was still in his pajamas.
"Key."
John took another bite of toast. "Tell me what it's for and I'll happily give it to you."
Sherlock growled and glared. "Key," he repeated.
"Heard you the first time." John let his gaze wander toward the refrigerator - probably wasn't any milk for tea, but it might be worth checking -
"Fine."
John looked up in surprise, waiting for Sherlock's explanation. But the detective didn't explain, didn't say anything, just shoved down his pajama trousers and what the fuck? John wasn't sure whether he'd said it aloud, but Sherlock read it on his face either way.
Sherlock glanced down at his groin, where his cock was currently locked into a shiny chrome contraption with little bars and a serious-looking lock dangling from below. "It's a cock cage, John, which I'm only telling you because you insisted on a damn explanation. I need my key back."
John knew the devices existed, had seen them occasionally for sale in the less seedy adult shops, but finding out that Sherlock actually used one . . .
He stood abruptly, threw the key down on the kitchen table, and fled to his room. And spent the two hours before his shift hiding from his flatmate.