A/N: Might be a shade crack-y, but I finished this several months after I lost my inspiration for it, so my train of thought was skewed.
He isn't really sure what they think he does. They probably don't even think about it, oh the ordinary people with their little brains and short attention should notice- he knew about the cabby, though they weren't there for that one, he knew about the fashion designer who was murdered by her assistant, he noticed these things and no one noticed him noticing.
It wasn't like Sherlock was surrounded by fashionable people- after all, John, while modestly dressed, still wore his clothes from before Afghanitan, and Mrs. Hudson and Mycroft were hardly the pinnacle of common dress. But what a person wore, what trends they catered to, could tell so much about a person. Like the assistant, who wore clothes over a year out of date- clearly ill-treated, not allowed to upgrade for fear she would outshine her employer.
So despite what John thinks (whatever he does think, that is), once or twice a month he goes out to purchase things he'd rather not request John get for him. Things like talcum to remove the scent from his shoes, or the special brand of deodorant he wore to avoid skin irritation. Or fashion magazines.
Suit jacket buttoned, sleeves rolled back, personal items tucked in a nondescript plastic bag, he'd visit the bookstore, collect the most recent fashion magazines for all ages. Men's wear, latest dresses, rising designers. He'd select around twenty each month, check out, and carry them home before John was finished at the surgery. Then he'd spend the next week or so, when he had free time alone in the house, pouring over the latest and most likely to become popular fashions.
Sometimes, certain clothes would intrigue him. That strap would create a noticeable dent, he would think of a handbag. Those shoes would cause a blister on the insole. And because it was pertinent to the Work, he would go out to test his theories. Sometimes he was able to handle the item in the store, to test the weight of the new dress with beads along the bottom which would affect gait.
At other times, he was less fortunate. Since the time he had started the habit, he'd ended up purchasing several various bags to test capacity, three pairs of fashionable pumps, and nine dresses with various irregularities.
They never questioned him about why he knew a certain shoe would rub at just that angle. But heaven knew he'd gotten enough strange looks from clerks when he bought women's clothes in his size. Good thing he didn't care too much.
Darling.