It is possible that Sherlock Holmes does not own any shirts of his own. Certainly Watson has never seen him purchase any. Watson has become accustomed to what Holmes likes to term their barter system. What this seems to mean is that Holmes' stuff is Holmes': Watson's stuff is ours, old boy.
Instead, Holmes will blithely raid Watson's wardrobe if there's some occasion which requires him to be well-groomed. Such as a night at the opera. "You won't enjoy it, Watson: it's Wagner, and when we saw his Dutchman you were distinctly unimpressed."
Luckily, Watson lost the habit of owning garments when he was in Afghanistan. One uniform is very like another, and dead men don't need a change of kit. He's not, on the whole, possessive. Not about things.
People, on the other hand ...
"Did you have some other companion in mind?" says Watson, who can be devious too, thank you very much.
"What? Oh," says Holmes, examining his fingernails, "I thought I might ask Mrs Norton to accompany me."
"Irene," says Watson flatly.
Holmes raises an eyebrow. "Is there a problem? I'm sure she'd be happy to yield her place to you. I'd merely thought that the opinion of a professional -- she is, after all, a contralto of some fame -- might be of interest."
Watson demurs. He doesn't actually care for Wagner. There aren't enough tunes.
"I assume you're going to change your shirt?" he enquires instead, already certain of the response. After all, the shirt that Holmes is wearing shows signs of having been present at various chemical experiments, not to mention that rusty brown smear on the sleeve.
"About that," says Holmes. "Might I --"
"Wait here," snaps Watson, because the image of Holmes rooting through Watson's wardrobe -- which currently harbours a very elegant linen shirt that was a gift from Mary, which Holmes would certainly select given the opportunity -- is immensely irritating.
He slams the door of his room a little more forcefully than the circumstance requires, and lets himself breathe; slow, deep, once. He is only a little surprised to find that he does not want Holmes to go to the opera with the woman; with Irene Adler, whose married name he will not use given her lack of respect for the state. He is rather more startled by the realisation that Holmes is his, whether the man realises it or not. Holmes is ...
Well. One kiss, and some schoolboy fumblings, might not be sufficiently concrete an attachment for Sherlock Holmes, cerebral and isolate as he is. Watson, though, tends to optimism: it's a good place to start. He hopes to continue very soon.
He thinks of the last time he accompanied Holmes to the opera. They took a box, courtesy of a grateful client. Perhaps said client had already heard Verdi's operas, and did not care to repeat the experience. The music had not been especially to Watson's taste: but the evening, ah, the evening was far from dull, with Holmes edging his chair towards Watson in order to whisper scurrilous gossip concerning the soprano, or possibly just to ... just to move closer. And then they had returned to Baker Street, and ...
Now Holmes is set to repeat the experience with Irene Adler. This simply will not do.
There are several clean shirts in Watson's wardrobe (and Mary's gift to him hangs pristine, awaiting some special occasion), but he ignores them. Instead, there is the shirt he was wearing yesterday. It will smell of his cologne: it will smell of his sweat. And, given his regrettable lapse of control after two hours of watching Holmes stalking around the flat in his shirt-sleeves, firing off brilliant observations and scathing remarks ... Well. Watson had at least assuaged his inconvenient urges behind a locked door: but it is very likely that the shirt he wore still smells of --
Indeed, there is a rigid starchy stain on the hem. Barely visible, and if he delays until the last moment before Holmes is due to leave, there will be no opportunity for his over-observant friend to detect the precise nature of that stain.
Watson raises the soft cotton to his nose and breathes deeply. Oh yes: it smells of his own body all right. And the notion of Holmes sashaying down the Haymarket with Irene on his arm, distracted by the lingering odour of Doctor John Watson, is ... remarkably arousing.
When Holmes returns from his night at the opera, senses full of Watson's intangible presence, wearing Watson's shirt (it'll smell of Holmes as well, now), Watson will need to clarify the nature of their arrangement, the quid pro quo of it, the way it's not really about things at all. He'll claim what's his: the shirt, at least, and maybe something more.
He finds he's rather looking forward to it.