What Mycroft noticed first, of course, was the sheet.

It was what anyone would notice, though he sincerely doubted that his instant and automatic intake and analysis of the relevant details would occupy anyone's notice, first or otherwise.

The sheet, catching the dappled sunlit green of the windowed wall, was folded unconsciously around his neck like a collar, and his little brother appeared to be focused entirely on the cryptogram he had scrawled across the desk in sharpie.

"Next time you get a sandwich," Sherlock greeted him, "don't eat the bread."

"I'm quite well, thank you for asking."

The younger Holmes deigned to face him. He rotated his sheet-swathed body in Mycroft's direction, his gaze stopping to take in the immaculate cuffs of his brother's trousers and then tracing a trail up his leg and across the engraving on his pocket watch, all with a half-smile that slipped down the lining of Mycroft's stomach.

And when he met Mycroft's eyes at last, his lips parted for a swath of teeth and his eyebrows reached for each other in the way that meant he was pleased, that this was good.

"I didn't need to," says Sherlock.