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"I. Love. You. You bloody twat," John growls.
"I know," Sherlock comments, still concentrating intently on the beaker before him.
"You know?" John squeaks.
"Yes. Discovered it earlier- you really should be more careful about how you do your laundry, anyone could te- well, alright, not anyone, but you're living with me, so I suppose it amounts to the same difference." Sherlock reaches blindly behind him and hums when John numbly hands him the pipette he'd been reaching for.
"Laundry?"
"Yeeees..." he adds a drop of liquid to the beaker in front of him and waits a beat, watching intently, "Thursday- the nice burgundy shirt, 400-thread count with the brass buttons, expensive, not an everyday shirt. There's a drop of grease on the tail- must've done it when your shirt was untucked, but this is a shirt that you don't wear untucked, therefore 'date shirt', therefore something you wear to dinner, so supposedly olive oil. Ah, but you don't wear a shirt like that untucked for dinner, so after dinner. After dinner, shirt untucked, with oil- sex.
"Sex with a man more than likely, judging by the matching stain on the back of the shirt, likewise in a place that wouldn't show unless it was out of the trousers. So, a shirt you wear to get off in. Slacks, new and well-fitting. Expensive also. A new development, not a longstanding relationship, something that happened recently.
"No longer than a few days, no less than one- you got dressed thus in the morning and did not change your clothes. During that time, the only person you had planned to see who was both male and new to your acquaintance was me.
"You wore an expensive shirt in a complimentary colour to your eyes, one you'd worn before to impress a male date, and went out and bought new, pressed slacks, all of which you put on in the morning, to wear throughout the day, not for a special occasion, but because you'd be seeing the person from the outset.
"Therefore someone who lives with you, therefore your male flatmate, therefore me."
John blinks at him, glances down, and then back up, head slightly cocked and with a dazed smile on his face. "I don't know why I even try to keep anything from you."
"Neither do I," Sherlock says amiably, spins on the stool, jerks John forward by the lapels of his jacket and kisses him soundly.
"I'd've felt better if you'd noticed me leering at you long before that, though," he murmurs into John's lips.
John smiles ruefully, "Well, we can't all be Sherlock Holmes, you know."