First is the tapping. It only usually takes one. The noise is a small stone hitting Sherlock's window-pane. The glass rattles in the frame, and wakes all in range. Of course, Mycroft can't understand why the window is even closed. The summer nights are unbearable, the humidity keeping every window open.
But not Sherlock's.
Mycroft listens as he rouses slowly, muttering to himself, and pushing up the glass. He leans from the sill (Mycroft knows because he's seen, and he's kept quiet about it) to hear a voice. The voice of his little blonde paramour.
"I can't keep you long," He says, and there's passion and soul and anticipation fizzing up in that voice, waiting to burst free. "Three hours, at most, but I have to work-"
"Ugh, work. Make it clear which you value more," Mycroft hears Sherlock climb out of the window, deft and precise. He hears the rattle of the gutter against the brickwork, and then the swish of the grass as feet pad across it.
Then, there's the soft, wet sounds of kissing that no brother should have to hear most nights. Sherlock's paramour brings a light, and a coat. They continue down the ground, Sherlock in a nightshirt and coat, and John wrapped up, ready as ever to be at Sherlock's side. Mycroft hears them laugh, and he hears them joke.
What he does not hear is the want and lust in the cherry orchard. Where Sherlock gasps against the tree bark and wails as if he's in pain, and God, he nearly is, by the way john's pretty little mouth works, in the subtleties of the tongue and of the lips...the warmth of the throat.
It's in the disused boathouse that the trouble really starts. Sherlock remembers the first time.
He'd said 'eyes on me', and that's just what John had done, removing his jumper slowly, and then his trousers, until he stood proud, naked in the dim of a lamp. it was strange, because Sherlock just looked at him for the longest time. As if he'd every idea what to do, but not the foggiest how to begin.
So John walked over in the dim, eyes on Sherlock, and unbutton the nightshirt, eyes on Sherlock. Until Sherlock was also clothes-less in the dusty air. The boathouse was cold.
"Eyes on me," John murmured, before he put that clever little mouth of his back to work. And Sherlock? Sherlock choked.
The first time was in June. It's the heat of July now.
Some hours later, Mycroft stirs again to the noise of voices, and to the faint shining light. Mussed, sated, Sherlock rattles back up the drainpipe and in through the open window. He falls into a dreamless sleep with a faint smile playing upon his lips. It's not as if Mycroft is a fool, he knows what has occurred, just as he sees it all over Sherlock's posture, in the little marks on his throat. Marks that other seventeen-year-old private students have much less of.
The first time Mycroft realized that, a thought had sprung to mind. He wondered what John would look like naked. In his dreams he saw the blonde adolescent standing in the doorway to his room, completely naked, and glowing against the darkness, eyes on Mycroft. He rolled over, back to the doorway. And let the illusion be.