What if when Sherlock gets back he goes to see John and John doesn’t even react just lets him slide seamlessly back into his life. And at first Sherlock thinks it’s kind of weird but he gets used to it, to John never bringing it up, to the way he sometimes just snaps and yells or breaks something because he figures, “That’s just the way he copes and I deserve so much worse.” Then one day Lestrade or Mrs. Hudson or someone shows up and starts talking to Sherlock and John goes all white in the face and says
“You can see him, too?”
And that’s when Sherlock realizes that the reason John never said anything was because he’d been imagining that Sherlock was still there the whole time. He never noticed that his hallucination had been replaced by the real thing.