Sherlock always knew. Whenever John tried to hide something from him, the events which followed the initial hidden event would always will themselves into a spiral of chaos, stopping at Sherlock Holmes himself. Afterwards, Sherlock would smirk and a witty one-liner would be announced to the living room of 221B. It nearly always resulted in John's stressed nature returning and a sigh would emit following his impromptu outburst.
But it didn't happen this time.
After a month of living without the consulting detective shadowing his chaotic life, John Watson felt broken. After two months John Watson left 221B indefinitely, following pleas from Mrs Hudson to try and forget the very man who'd made his life a living hell. But of course, in a good way.
Then, after six months of nothing but pain and misery, John was moving on. He had a job. A house. Even a love intrest. His life was falling into some sort of unwanted order; it was evolving.
But two months after that, John saw something that brought the events of his past life, or so he called it, back with the force of a wrecking ball. He had seen him. Alive.
And so John Watson had something to hide.
Note: I can't promise frequent updates for this but I'll try!