Written as an attempt for humor. Do not take this too seriously.
John came home one year after Sherlock was announced dead. Still living in 221b, he was shocked out of his skin when Sherlock was found reading a book hanging upside-down form his feet on the ceiling. His mind short-circuited, and John asked probably the most unimportant question he could ask at the moment: "Sherlock, what are you reading?"
"Would you prefer a lie or the truth?" the brunet answered, eyes never leaving the page.
"... Am I going to freak out either way?" John went into the kitchen to make himself a cup of tea and put away the milk and bread. This wasn't the first hallucination he'd had about Sherlock being home, and it probably wouldn't be the last.
"There is a high possibility." Sherlock's baritone voice drifted from where he was still reading.
"Tell me the lie." John demanded.
Sherlock sighed. "I'm reading an instruction manual on how to make a bomb."
His eye twitched and he felt like freaking out. "Now the truth?" John almost didn't want to know the actual answer.
"Mycroft dropped off all his old porn books. He told me they would increase my deductive capabilities by a hundred fold if I could see tells enough to recognize the ending by the third chapter of the book."
Cue the second short-circuit that night. THAT WAS WORSE! WHY DID HE ASK FOR THE TRUTH? "... What?" he croaked, his voice suddenly very hoarse.
"Don't tell me you actually need me to repeat that." Sherlock rolled his eyes.
"No, but porn?" John asked incredulously.
"Yes, porn." Sherlock nodded.
John spluttered. "... Okay, putting aside the porn for the moment, how are you still alive?"
"I faked my death, obviously." Sherlock sighed. "John, if you're going to ask boring questions, I may as well leave."
The blond shrugged. He had better things to do than endure Sherlock's smarminess. "How was your day?"
"Taking down the rest of Moriarty's network, y'know. Same old." the pretty brunet shrugged. "Gotta get back tomorrow, so we'll only have one day together. Bugger for that, but I'll be coming back in around a year. Don't wait up."
Ignoring that. "You're gonna have head rush when you come down. You do know that, right?"
"WHY ARE YOU READING UPSIDE-DOWN?!"
"Uncle Alucard suggested it. It helps with controlling the flow of your blood against gravity." Sherlock answered, casually turning a page. "Don't as about the 'Alucard' part. It should be obvious to you by now that everyone in my family have strange names."
John couldn't deny that. 'Mycroft' and 'Sherlock' were bad enough, but 'Alucard'? IT WAS LITERALLY 'DRACULA' SPELLED BACKWARDS! "Do you have another sibling?" He'd been grilling Mycroft about another sibling because Mycroft wasn't doing much as a support network, not the same way Sherlock had been. If he had a sibling, they could share stories over booze and stuff...
"Eurus? She's locked up in Sherrinford. Unimportant. She just burned down a few things, killed a few others, and could manipulate people to do exactly as she wanted."
"YOU PEOPLE ARE ALL INSANE!" There go the booze ideas. (Dear god, he was turning into Harry.)
"Well, she's locked up with the express orders never to give her human interaction. It should be fine for another few years." His eyes lazily danced from one side of the book to another.
"YOU HAVE A MURDERER FOR A SISTER AND YOU DON'T EVEN BAT AN EYE!"
"No, I don't. I've known about her since birth, y'know. No amount of reprogramming can completely erase a childhood."
"FUCK YOU, ILLUSION!"
"I'm not an illusion, and if you had any sense you would recognize that."
"MY ILLUSIONS ARE AWFULLY REAL!"
"Perhaps you should see a psychiatrist about that."
"I AM SEEING A FLIPPING PSYCHIATRIST!"
"They don't seem to be working."
"No, thanks. I have other ways to work off sexual frustration rather than bedding my flatmate." Sherlock flipped a page.
John picked up an ashtray and chucked it at the brunet's head. It bounced off Sherlock's forehead and landed on the carpet.
"See, not an illusion." Sherlock finally lowered his book. A small red mark on his forehead bloomed not long later, and John nearly cried with relief.
The two flatmates talked to each other for hours. Sherlock was to remain a secret from the public (not from Scotland Yard, though, because the brunet couldn't stand it if there were no cases to work on). Mycroft was obviously informed he was back and still alive (hence sharing his porn collection).
Sherlock was gone the next day. But it was okay, because this time, he had proof he wasn't crazy.
Sherlock's number had been reprogrammed into his contact list, and Mycroft's old porn novel rest peacefully on his arm chair.
'I'll be a while. Get some sleep.' Sherlock told him in a text once John assumed he was reasonably safe enough to consider it alright to text him.
John didn't dare text back. Instead, he went to his room and went to sleep.
No sense waiting up.