He's about to sweep out of the morgue when he notices the soft smile Lestrade and Molly exchange across the room, and he pauses in something akin to bewilderment. His first instinct is to delete the unexpected bit of information; he's somewhat uncomfortable about the notion of his friends as sexual beings, and therefore chooses to ignore the concept half the time.
However, that's easier said than done when two of his friends are in an intimate relationship with one another. The evidence is there whenever he stumbles upon either of them, and he can't quite work out the reason why he feels distinctly uneasy about it.
It's this nagging feeling that prompts him to address the matter with his brother a couple of weeks later. Mycroft pours himself another glass of thirty-years-old Laphroaig whisky and stares pensively at the mantelpiece, leaving Sherlock to wonder for a painful moment.
"You… and Lestrade? Are you kidding me?"
"I'm surprised you didn't figure it out earlier, brother mine."
"Never mind that. What happened to your motto 'Caring is not an advantage'?"
Mycroft shakes his head wearily. "It is not. And yet even I am only human after all."
His frown deepens as he struggles to reconcile the idea of his emotionless brother with something as ordinary as feelings; he gives up soon enough, lest his brain implodes under the pressure.
"Why did you let him go?"
"All hearts are broken, Sherlock. Now he's just somebody that I used to know."
"This is the most ridiculous thing I've ever heard from you," he claims with conviction, reaching for the bottle of Laphroaig and an empty glass.
That night he lies in bed thinking of Mycroft and Lestrade and Molly and sentiment, until his mind is running round in circles and he can't make sense of anything he's always held as true.
His brother doesn't do sentiment, or that's what he's been led to believe. Love is a chemical defect found in the losing side and yet, he finds himself questioning his occasional slips with women – Irene Adler, Janine, even Molly in a way. And Mary, always Mary, because she's John's wife and Sherlock's soul sister, and everything is so complicate and simple at the same time.
"Why did you do this to me, brother mine?" he mutters to the room at large, but nobody is there to answer as he surrenders to sleep.