The Answer

So this came from a Wholock prompt on Tumblr... First part, there will be more.

It starts when John decides to tidy up after Sherlock. Up until this point John had been under the impression that Sherlock was a messy person. That was until he saw Sherlock's bedroom. Pristine, military almost. It reflected a childhood of being treated like an adult.

It seems to John that Sherlock is only capable of keeping one room of the flat tidy at a time. Sherlock still leaves his towels damp on the bathroom floor. Sherlock still refuses to do the washing up- ever. Sherlock still puts his shoes up on the coffee table. And Sherlock still has piles and piles of books, knick-knacks littered around Baker Street, and unopened boxes of who-knows-what lurked in corners. Their absence from Sherlock's room is how John believes he keeps it tidy.

So on a Thursday when Sherlock had somehow managed to get a case before John had woken up, probably having not slept since the night before, John decided to organise the flat. He begins hefting the books into the empty shelves, even deigning to alphabetise them- though he suspects that Sherlock would much prefer the Dewy decimal system. He couldn't bring himself to care.

It was only when John finished with the books and started opening up neglected boxes that he almost regretted touching Sherlock's things.
Sentiment wasn't something John imagined Sherlock feeling a lot of, and yet here was a photo album, filled obviously with people Sherlock cared about. The strangers smiled at the camera, happy. Sherlock was in a few of them, even fewer of them had evidence of him smiling. Looking back through them John was struck with the realisation of how little he knew about Sherlock.

The box with the photo album held various other things, some of which John couldn't recognise as something Sherlock would ever own. A carved wooden box bearing intricate circular patterns hid beneath several more photo albums, the contents of which seemed to call almost to him.
A gentle thrumming under his fingers, almost as if someone is singing in such a bass tone that human ears can't hear it, only feel it. He traced a few of the circles, the thrumming becoming stronger.

John replaced the box, steadily ignoring the beat he could still feel under his skin. He closed the box and continued cleaning and sorting Sherlock's other things.
That cardboard box stays closed for a week longer.

That week John notices a change in Sherlock. He sees Sherlock's quiet contemplation punctuated by the occasional quirk of his head, ear cocked as though he could hear something that John could not.
He would be mid-flow, expelling his doubt at the origins of a particular burn mark on a victims neck, when he would stop. A stop so small and so subtle that no one else noticed. His eyes grew a little distant, as though focusing on something that no one else could see, before snapping back to the present, the case, the work.

It took a week for the tension to become overwhelming, for the thrumming bass under John's skin to become palpable in his every move. It took a week for John to drag the covered box out once more, to unearth the carved wooden box from beneath the photo albums.
He held it reverently, the thrumming becoming rhythmic, he could feel it coming from the box. His hand hovered once more over the clasp keeping the box shut, so close, so very nearly-

"John? What are you doing?"

The rumble pulled John from his reverie, and he dropped the box in shock.
"Sherlock, I didn't hear you come in."

The look on Sherlock's face was blank.
"John Watson, what are you doing with that box?"
John gathered to box to himself again, the thrumming had become louder, deeper, more rhythmic. Almost as though it knew Sherlock's presence. John could feel it.
"Nothing." He replied breathlessly. "I was just tidying some of your things-"
"Give it here."
There was no mistaking the command in his voice, touched by something that John couldn't quite place. Concern? Panic?

A small part of John's brain saw the outstretched hand, and didn't want to hand over the box. That part was silenced by the look in Sherlock's eyes. He held out the box.

The moment the box left John's fingers the drumming stopped. Sherlock's eyes widened. He can hear them, John thought, he can hear the drums.
Sherlock's deft fingers flicked the catch of the box and threw back the lid in one fluid motion. As though he did it every day of his life. John was gone to him now, all he could see was the box in his hands and its contents. He dipped his fingers into the box, and gazed reverently at the dull, gold pocket watch.

"Sherlock?" John could see that his voice had dragged Sherlock back from a gaping precipice. Sherlock narrowed his gaze.
"Where did you get this?"
"It was in amongst your things. Why? Where did you get it?"
"Haven't a clue. Never seen it before in my life."
Sherlock's attention went back to the pocket watch, more in control than before. Clearly trying to glean something from it's surface. John caught the wooden box as Sherlock cast it aside.
"Unimportant." He muttered.

John watched quietly as Sherlock paced with the watch in hand. He flipped it over and over, swung it suspiciously, smelt and licked its surface, making a face at the metallic tang, eventually putting it to his ear.
"Broken." He said shortly, tossing it into John's lap. "Uninteresting. Boring."

John glared at him. He didn't recognise it, and here it was in amongst his things. And that was boring? John turned it over in his hands, noticing that it had the same circular patterns engraved on it as the wooden box also sat in his lap.
Sherlock had long retreated when John stopped inspecting the watch. Had sashayed off to start another toxic experiment in their kitchen probably. The silly bugger hadn't even bothered opening the damn thing. He turned it in such a way that the dial used to alter the clock's face was beneath his forefinger, the warm weight of it settled in his palm. It was only then that he noticed that the sound of drums had stopped.

John clicked open the watch.

The hush that fell just before it happened was unearthly. It was as though all sound had been sucked away, until there was only the sound of falling rain, and faint drumming. And it was coming from the golden mist that had spiralled up out of the pocket watch in John Watson's hand.
It paused, and John's mind almost believed that the mist was sentient, before rushing in a great blur of golden light into the kitchen. John heard the sound of glass breaking. And the drumming had stopped.

The rain however suddenly seemed to be beating against the windows of Baker Street. It had become much darker. John sprang up from his chair, the wooden box fell to the ground, the watch still clutched in his palm.
"Sherlock?" He called, filled with trepidation, "You alright?"

The sight that greeted him when he reached the kitchen was like any other time he had glanced into the kitchen. Other that the broken beaker on the tiled ground, there was nothing out of place. Sherlock has his back to him, stood with one hand curled into his dark hair in a rough claw, the other hung limply by his side.
The golden mist had gone. Sherlock wasn't answering him.
"Sherlock?" John said again, agitation rising in his voice, "Did you see that-"
That was when Sherlock turned around, and John barely recognised him.

His usually sharp and bright eyes were dull, and filled with tears, gazing past his empty hands to the floor below. His gaze finally raised to John's, and he stared at him as though looking at a stranger.
"Oh John," he whispered, his voice nothing like any tone John had ever heard him use before, warmer and yet miserable, fond and somehow chiding gently, "what have you done?"