Reapersun drew it. Diane Duane suggested it. All I did was type it out.
(See this story on my AO3 account for attributive links.)
It was only a suspicion at first. Not because John was as unobservant as Sherlock made him out to be, but because the idea was downright irrational.
It was all too true that Sherlock was aware of his own attractiveness, he used it as a tool all the time, and he was just enough of a ham to employ dramatic clothing when necessary. So when he popped the collar and swirled the skirts of his massive black coat John just rolled his eyes and kept his grin to himself.
When he overheard a discussion that revealed that one of the first things Sherlock had demanded on his return from being dead was his coat, John chalked it up to Sherlock being his usual demanding self, with a side note for later pondering about possible trauma and the comfort of familiar objects.
When he heard Mrs. Hudson talking to the thing as she sorted through the hall closet, John simply put it down to her mildly batty ways.
But when he saw Sherlock take a hit from a cornered murderer wielding a handgun - and where the hell the woman had picked it up - and yet when all the fuss was done and John got a chance to look Sherlock over, and there was only a massive bruise under unmarred cloth…
When Sherlock was out watching a house and waiting for the owner to emerge, and John walked past him three times in broad daylight without seeing him once…
When no matter what it was they needed, Sherlock seemed to have it in his pockets, even a wrench that technically shouldn't have fit…
Well. John was a rational man, but he also knew that when presented with new information, the thing to do was reassess.
He started paying more attention. And saw how Sherlock's coat often flared dramatically even when there wasn't enough breeze to stir hair. How when Sherlock stalked away from his brother in high dudgeon, the hem flipped just a bit in Mycroft's direction, as if gesturing rudely.
How the damn thing managed to get drenched in beef blood during one of Sherlock's little "experiments" and yet was pristine the next morning even though the nearest twenty-four-hour dry-cleaners was clear across London.
None of those things was enough to make John do more than eye the coat uneasily; he'd handled it a time or two in the past and it had been as inert in his hands as any ordinary garment. Still, under the circumstances, he didn't quite feel like picking it up to read the label.
It seemed rude, somehow.
It wasn't until the canal incident that John was convinced, though. A damned boat chase down the Avon just outside of Bath, at night, in bloody December, with people jumping from boat to boat and Sherlock waving a barge pole around - how had he found a barge pole - and the boat had shifted just the wrong way underfoot.
Of course John could swim. But the water was half-skinned with ice and the wind was blowing hard, and even after he'd hauled himself out (with the help of an iron grip on his collar) he could barely stand for shivering. "Don't let them get away," he told Sherlock, or tried to, and heard the frustrated growl before Sherlock stripped off his coat and dumped it over John.
He was blinking water out of his eyes, so he could be forgiven for thinking that Sherlock took a second to snug the coat around him. Except that when he got his eyes clear Sherlock was at the far end of the boat and just leaping for the next one, and meanwhile John's arms were in the sleeves when he was sure he'd had them wrapped around his torso just a moment before.
And the amount of heat that was soaking into him made no sense unless Sherlock had somehow lined the thing with an emergency blanket.
John steadied himself and considered charging after, but he was still shivering hard, and then people started falling off the other boat - or jumping, it was hard to tell - and he decided somewhat woozily that Sherlock had things well in hand.
He looked down, and had to snort. "Buttoning yourself is a bit much, don't you think?" he muttered.
The coat gave a subtle ripple, almost a shrug, and wrapped him tighter. A crash came from the other boat, followed by cursing in a deep voice.
They both sighed.
John didn't say anything, of course. That way led madness, or at least extreme irritation. Nor, despite some curiosity, did he try the coat on deliberately. It was Sherlock's, after all.
But he would hang it up neatly when he found it tossed over a chair, which he frequently did, and maybe give it a pat as he did so. And if it patted him back, two comrades in the ongoing chaos that was Sherlock Holmes, well -
He wasn't going to protest.