Disclaimer: I own nothing, not even the idea for this story.
A/N: I decided to join the Sherlock Movement, so here's my contribution. :)
Obvious spoilers for The Reichenbach Fall and possibly other episodes.
The first six months without Sherlock passed before John could even realize it. After visiting his grave with Mrs Hudson and asking for a miracle that didn't – couldn't – happen, he refused to go back to the flat.
He stayed with Harry, willing to forgive her if it meant he didn't have to go back to 221B alone. After six months sleeping on her couch though, his sister finally put her foot down and kicked him out, ordering his to go home and get some closure.
With no where else to go, and no where he could escape to since he'd been fired from the clinic, John, for once in his life, listened to his sister and left. There was nothing to take with him; everything he owned was still at 221B. Mrs Hudson had rung him to say that she wasn't going to get rid of anything, not even Sherlock's chemistry set.
For some reason he'd expected things to have changed, but the stairs still creaked as he stood on them, Sherlock's things were still thrown about on the floor, the stupid Deerstalker hat was still on the table and the yellow face was still painted on the wall with odd bullet holes decorating it.
It was that pointless smiling that started everything. Ideas began to form in his brain, crazy risk taking ideas, but they took hold of him and wouldn't let go. Without even thinking about it, John began to move through the flat, throwing every object that came into his reach and searching with a crazed frenzy.
"Doctor Watson!" Mrs Hudson's shocked voice didn't slow him down in the slightest. "What are you doing?"
"Searching Mrs Hudson," John grinned manically, reminding Mrs Hudson of Sherlock when he was on a case.
The resemblance left her breathless and for a few seconds she could believe that somewhere in another room Sherlock was also searching for something that would only serve to confuse her but that he understood perfectly.
"Got it!" John exclaimed, holding up a can of yellow spray paint.
Not even looking at the trashed room, John strode out of the room with the can of paint. For the first time in months, Mrs Hudson felt like John would really be okay without Sherlock, it made her feel better, and she didn't even have the heart to chastise him when he tested the paint on the wall while going down the stairs, leaving a streak of yellow trailing behind him.
John didn't know where he was going, but the destination didn't matter, the only thing that mattered was that he had something to do that would make him feel better and cause a little of the excitement that had been missing in his life since Sherlock had died.
He stopped in an alley next to St Bart's Hospital; the hospital that Sherlock had ended his life on. Refusing to dwell on memories that wouldn't help, John marched into the alley and raised the can of spray paint.
His tag wasn't artistic, but that had never been the point. In yellow letters across the wall, John wrote 'I Believe In Sherlock Holmes'. Before the paint could even dry properly, John got out his phone and took a picture of the tag. Unwilling to leave it like that, on the wall opposite he wrote 'Moriarty Was Real'.
On his way back to the flat, he bought more cans of the yellow paint, checking to make sure they were the exact same type from The Blind Banker case. His life seemed to have a bit more purpose to it, and if it was breaking the law even slightly then it was even better.
Walking straight into 221B, he turned his laptop on, while it was loading he began to make himself a cup of coffee, forgetting that he hadn't been there in months and still expecting to find milk in the fridge.
"Mrs Hudson! We're out of milk!" John shouted, falling back into the habit of using plural speaking when talking about himself without realising it.
"I'm not your housekeeper dear," she said automatically, but still brought a bottle of milk up to him and placed it into the empty fridge.
As usual, she was ignored and soon left to go to her own room, leaving John alone. Logging into his blog, John was slightly surprised to find his comments page flooded; comments ranging from people supporting him, to people asking whether he was in on the fraud. John downloaded the pictures of his spray-painted tags, but didn't write anything down; just knowing that somebody might see them was enough.
Over the next four weeks, John added his yellow tags all over the city, growing bolder every day. Other people added their own tags to them, mostly the same message, but occasionally John would see a painting of Sherlock or sometimes just the Deerstalker hat.
His message wasn't just over London, people sent pictures in of tags they'd found (or done) from all over. Henry Knight even posted a picture of the words 'I Believe In Sherlock Holmes' in big red letters written over one of Baskerville's wall.
Sherlock had been dead exactly a year, but his name was more famous than ever, the tags had grown into something else; t-shirts, badges and stickers were being sold with John's tags written on them and people were still joining him in his pursuit to paint his message across the world.
On the date of Sherlock's death, John took his yellow paint and went to the police station. It was luckily empty, but just in case he was caught on any cameras, John put his hood up and then wrote over the front window of the building 'Moriarty Was Real'.
"You know," a recognizable voice said. "I may not be an inspector anymore, but I could still arrest you for that."
"You could," John agreed carelessly, turning to face Lestrade. "But I don't think you will."
"It seems you're right," Lestrade grinned. "I should have known you were the one responsible for the tagging."
"You're not going to tell me what I'm doing's a bad idea?" John asked, slightly curious.
Lestrade shifted guiltily, pulling a can of red spray paint out of his coat pocket. "I'm afraid I'd be a hypocrite if I did."
"Breaking the law," John teased. "Sherlock would be proud."
"Well then," Lestrade straightened up. "Let's make him prouder yet."
He took a key out of his pocket and opened the door to the police station, gesturing for John to go first. Grinning happily, they began to write over every wall and desk after spraying the cameras. John took great joy out of tagging Sally Donovan's desk a bit more extremely than the others.
"That was fun," John commented once he was back at 221B with Lestrade and Mrs Hudson. "Do you have keys to anywhere else?"
Lestrade dangled a bunch of keys in front of them. "I stole them out of Anderson's desk."
"Then let's get to work," John picked up his can and made his way down the stairs, knowing, just like Sherlock had known, that they would follow him.
I BELIEVE IN SHERLOCK HOLMES