They were surrounded by flashing red and blue lights and the faint smell of cigarette smoke. There was a dead body less than 100 meters away, packed away in an oozing body bag, and they were certain they heard fighting even nearer than that. It was disgusting and uncanny and irregular and so them that neither of them felt the urge to recommend their first shared kiss happened anywhere but there- in an ugly, dark alleyway, hidden from invasive eyes but, simultaneously, right in front of them.
Everyone saw but no one did. No one saw them, but the second their lips interlocked, everyone around them knew what was happening. Maybe that's what happens when a know-it-all is met with a sweet and savory kiss from the man even he never envisioned himself with- although, to be fair, he never envisioned himself with anyone. So maybe, when he was caught off guard for maybe the third time in his entire life, his seemingly superhuman ability to know everything happening anywhere- past or present- was passed on to the nosy policemen and women that he both despised and loved- But only for a moment, because the second he was aware of what was happening- the second he regained control- he took back his perfectly human ability to over-perceive... and focused it all on John. The way his hands shook against his face, and how he was otherwise still, flush against the other boy's chest; How his eyes were closed so tight, and his breathing was labored and nervous.
For once, Sherlock understood why people were so scared about a simple kiss-- because he was the exact same way.
His hands shook as they took hold of the slightly-shorter man's hips, and his body was perfectly still from his shoulders to his feet; his eyes were equally squinted, and his breath was nearly gone. He echoed the other boy's, well, everything. His reaction, perhaps. He doesn't know. He finally focuses on exactly what's happening- well, he knew before, but knowing and understanding (as he's explained a plenty of times before) are very, very different concepts- and realizes just how sweet everything is at that moment.
Sure, it's dank and disgusting, but it's also sort of pure- sort of innocent; Kissing John feels like a memory-- One he's locked away. He knows he's never experienced this before, but he just cannot shake the feeling that sharing such a moment with John just feels so... familiar.
Then it hits him: Home.
Maybe, kissing John approximately 314 feet away from the rotting corpse of a 47-year-old man, hidden away in a dirty alleyway... Maybe that's his home; His own grotesque, fucked up home.