Out of all the things John had to get out of the flat that was only really used by Sherlock, the one thing he wouldn't even dare touch was Sherlock's violin.
John threw out every experiment, every useless piece of paper and even changed the curtains, that seemed to smell of cigarettes and hair soap. Every time John would prepare to leave with a large box of things in his arms, he'd look to the side of the room, where his friend used to compose and analyze, and at the wooden instrument and then shake his head and shut the door. He really couldn't bring himself up to taking it with him and selling it.
Sure, he decided not to part with a few of Sherlock's other things, knowing that if he got rid of them, Mycroft would have a fit and go through too much trouble to get them back. So there was still a skull on the mantle and The Periodic Table was still hung on the wall in the extra bedroom. Sherlock's microscope had been set in his now empty room, on his dresser.
John knew Sherlock's violin wasn't on the list of things Mycroft wanted to keep, he didn't know why and he thought that he'd feel a lot better about keeping it if it was, he knew that he could use the money if he sold it but he couldn't get himself to even take a step towards it.
When John met Mary Morstan, about 2 years after Sherlock's death, his life seemed to brighten again and he began living his life again. He got coffee and caught up with Greg, who he hadn't spoken to in over a year, and agreed to trying at cases again. He tried, only to be reminded of Sherlock and feel upset. He tried to push through it and sure enough after the first case, he was helping Greg out every few weeks and he found that he missed the thrill of a case. Every few weeks he'd be buried in case files, and found that his life was still a little broken and nostalgic but was coming back together slowly with his Mary, his work at Bart's with Molly Hooper and the cases.
One day, Mary stayed overnight with John at 221b and sat in Johns chair in the morning, not knowing or caring about whose chair was who's or any nonsense like that, and so John carelessly sat in Sherlock's old chair. He coughed lightly at the dust surrounding him and was suddenly reminded on who's chair he was sitting in and why it was so dusty, he relaxed onto the back and pushed the thought away. Mary then pointed behind him and said,
"I didn't know you played violin?"
John froze and coughed again stiffly.
"I don't." He said softly, "a friend of mine does- did. I should say." He rolled his neck up to look at her and she nodded.
"Oh I see." She replied with a frown. "I'm sorry, love. Drink your coffee."
John shook the conversation away and gulped his coffee down before returning back to his room to pack for his weekend and his case with Lestrade. Mary left soon for work soon after and before John walked out the door to leave, he glanced at the violin then sighed heavily, noticing the cursive "SH" on it's side and the dust that had collected on the top.
"Sherlock would be disappointed in my lack of care for it." John thought to himself and before he could even process what was happening, he set his bags onto the sofa, took some cloth that was in the violins empty case and began cleaning off the dust that had gathered for two years on the untouched violin. After he finished cleaning it, he realized he was 30 minutes late, and Lestrade would be wondering where he was. He put the now beautifully clean violin back on it's stand and gathered his things to leave. He'd tell Greg he fell asleep.
John returned home from solving a difficult case, that really seemed to be too simple in the end. He constantly thought of how Sherlock would have solved it within 5 minutes, and it took him 2 days. John sighed as he paid the cab driver and began walking up to his door, only to find it already unlocked and cracked open. John parted his mouth and whispered an "oh no." And pushed the door to go inside. John stayed silent,and heard a faint light hum.
"The violin? What the hell?" John thought with a confused look on his face. "Why on earth would someone break into my flat and just play the violin?" John silently walked up the stairs, trying to put his finger on the piece the violinist was playing, but couldn't, although the piece was familiar.
John held his breath and pushed open his door, to see a slim dark silhouette standing in front of the window, performing beautifully a song John knew he knew but couldn't name. He didn't dare breathe out a word, or breathe out at all and only let out an audible sigh after the silence was broken as the piece ended.
"Thank you for cleaning it for me, John. It's as if you knew that I'd be back." Said the low, soft spoken, and all too familiar voice.