What if after Sherlock comes back, that John makes a habit of grabbing his wrist and checking for a pulse as subtly as he can. Sherlock knows of course, but lets him. It makes John feel better, more assured, so he doesn’t mind it. Soon though, it becomes a thing John does without thinking, and for the rest of their lives, John will check for a pulse in Sherlock’s wrist. Just to make sure he’s still real.