A.N. I read a lot of fanfictions where Sherlock kept the Coat (it definitely deserves the capital letter) on, no matter the weather. Sometimes even no matter the season. Then I realized he's fully justified...
Disclaimer: nothing is mine. Obviously. ;-) I'm not BBC staff, and even less Arthur Conan Doyle. Betaed by the wonderful Ennui Enigma (as well as my last oneshot, sorry about forgetting to mention it there, dear!).
When John Watson meets Sherlock Holmes (and is momentarily transfixed by the man...though it's hardly his fault: it's practically a close encounter of the third kind) he thinks nothing of the other's coat. The weather is, after all, cold.
If he notices the coat, after a while, he's only vaguely annoyed. Trust Sherlock to find the perfect way to combine weather-appropriate dress and attention-drawing are simpler attires, but these do not allow for collar-flipping and cape-whooshing. If Sherlock wants to look like a model on holiday, it does not concern John. It doesn't even register on his radar, unless he has a date and is trying to dress up (and feeling like he's failed miserably, at least in comparison).
Months go by, the weather gets steadily warmer (like it should), and John tucks away his beloved jumpers. He's not keen on getting heatstroke, thank you very much. However, Sherlock's coat stays firmly glued to his back. Now, he doesn't look cool anymore. He just looks way hotter than he should be.
John's mind startles and rewinds. He didn't mean it like that! He's a doctor, pondering presumptive body temperature. So professional he's never even realized the possibility of a pun when he had to treat beautiful women with heat exhaustion (and it happened, some women are incredibly dumb). So why the epiphany now? Somebody up there hates him...or at least likes making John uncomfortable. He isn't attracted to Sherlock. Not in any 'hot' way, at least. He's obviously enchanted by the man's brain, but that's a whole different matter. Anyway, he needs to get his brain back on track. Before his mind-reading flatmate picks up on that and sneers. Right. The coat.
With anyone else, John would suspect holding onto it meant that it was some sort of security blanket. Maybe he would even tease his friend a little about it. But let's be pragmatic, this is Sherlock. Sentimental reasons... psychological reasons altogether (he doubts this, sometimes) are thrown out the window. No, it's much more probable the bloody genius didn't alter his attire because he has simply not noticed the weather change. Unless things pertain directly to a case (like with the Study in Pink), John wouldn't be surprised if Sherlock happened to delete menial facts like these. And Sherlock is only too used to ignore his transport's (and why can't he call it body like everyone else?) discomfort to take the hint from that.
John is even less keen on having to nurse Sherlock back from a heatstroke than he is on having one himself. So, next time they are out, he simply asks the man if he wants to take it off. Hell, he even offers to carry it. He doesn't mind playing porter...much, since being around Sherlock has its definite perks.
Sherlock refuses with a curt "I'm fine, John!". Well versed by now in Sherlock-to-world translation, he realizes this is not the 'I'm focused on something else and hence fine unless proved wrong by collapse' fine, it is the 'I am comfortable like this and I don't care if it's not normal for the rest of the world' fine. Surprised by the realization, John stops seeing and starts observing. He observes that despite the temperature, Sherlock is not sweating. He is not, actually, presenting any signs of overheating.
Conclusion: Sherlock is colder than most people. Again, the double entendre is rather obvious, as well as undeniable. What's more unexpected is that the literal sense of it is true.
John runs the evidence in his head. Nothing too worrying, otherwise it would already be evident by other symptoms. Still, if he doesn't keep track of what happens to Sherlock's transport, who will? Not Sherlock, that's for sure.
It takes his friend leaving yet another meal barely touched (did someone make a sign saying crime chasing only at mealtime?) for him to get it. It's obvious, really. Sherlock is the most underfed person John has seen this side of the English Channel (leaving out fashion industry workers). The word 'calories' does come from the Latin word for heat...and Sherlock surely has none to spare for his own thermoregulation. He's bound to get cold.
If John ever managed to make him eat like a human instead of a baby bird, he might even get to see a coat-less Sherlock outside the flat. That would probably startle a photographer. John knows this particular utopia is only a dream though.