Nothing leaves John Watson so bereft as when a lover leaves. Even if it's temporary. Even, as Sherlock Holmes had done, if the lover has simply been distracted by a lead he's thought of mid-coitus, and he's run off to follow it up.
Leaving John on the bed, hard as hell. Fortunately he has a mental image of Sherlock at the precise millisecond he figured out the missing piece. The moment when he was under John, prick against John's stomach as John fucked him deep. That moment when he opened his eyes wide enough to let the whole universe inside.
Sherlock was apologetic, as much as he could be when he had an epiphany on his mind. In thirty seconds he was dressed and out of the room, leaving John to take himself in hand.
Soft skin covering hard intent slides under John's palm as he imagines Sherlock at that moment of recognition. The moment when he dipped his mouth to sink into John's, apology for the hasty exit he was about to make. As he slipped out from beneath John's body, the long slide as he pulled away from John's cock gave the man left behind plenty to focus on.
The sheer brilliance shining in his eyes as he haphazardly tugged on slacks and a button-down shirt. He'll be three blocks away from 221B Baker St. before he notices his open fly and his shirt, one button off. And he'll put himself together properly in the cab, because otherwise the imperfection of his attire will distract him from his realization.
John registers pain, he's clutching himself so tight. God, that man winds him up and never let him down. Even when he left him alone and unsatisfied, his brilliant mind carries John through. The way Sherlock constricted around John's cock at that moment of realization... John felt him reach a conclusion no other man could, the mental orgasm of a thousand pieces fitting into one gorgeous mosaic pointing to a single guilty party.
John's breath draws in and in again as he comes, hard, to that transporting glorious expression on his absent lover's face. As he rides out the lonesome glow of a solo orgasm he takes comfort in the knowledge that in the morning, over a breakfast Sherlock will cook as recompense for the night's abandonment, he will hear the genius solution to the problem of the night. He will be the first to know, because he matters that much, and he knows when to shut up as tight as crystal and listen as Sherlock explains it all to him and no other, legal and illegal, from the beginning to the end.