The line between living and dying was surprisingly easy to miss.
John Watson didn't notice when he crossed it.
It was at least partially his own fault, his damned stubbornness to continue living, despite being shot. So he didn't really notice when he made the transition from his body to his new form, whatever the hell it was.
(He tried not to think about the state of his body, rotting in the ground somewhere. It made his pseudo-skin crawl.)
No one else noticed. (His body was never found, so no one had to face the paradox that was two John Watsons.)
He went back to London.
But he couldn't help but feel like he was fading.
("Nothing happens to me," he told Ella, and it was true. More true than she could possibly imagine.)
He was surprised Mike Stamford saw him that day, he was feeling particularly translucent.
But then there was Sherlock Holmes, and John had never felt so... solid.
The time he spent with Sherlock, solving crimes and being exasperated with body parts in the fridge and bullet holes in the wall, was the best time he'd ever had.
He almost forgot that he was dead.
Still, no one else knew.
For all of Sherlock's genius and knack for seeing through everything (that was funny, John could almost laugh, because literally, see through) he didn't seem to notice that his flatmate and best friend was different.
Perhaps he chose not to.
Or more likely, John mused, Sherlock was too scientific, and too practical to even consider the evidence he may have been seeing.
Ghosts, after all, were a scientific improbability. (But not impossibility.)
But then Sherlock jumped (why, why did he have to do that, the stupid prat. I was already dead, I could have taken a bullet or whatever, there was no need for him to be so foolish and now look where it's gotten us, both dead, but only one still here) and John lost his anchor.
It was just so hard to stay, so hard to remain solid enough to touch things, to speak, to appear to people.
He just wanted it all to stop.
So he went to therapy with Ella (fat lot of good that did him), and afterwards went with Mrs Hudson to the grave, touched it, like that would help him in some way, and then returned home.
There, he just stopped.