I can't believe some of the questions on here, I thought everyone knew Hermits could fly but only if the salinity and lighting levels are right.
Sorry back to the real world. Hermits will climb up the edge of the tank if they can get a grip on the silicone (at least mine do). They really don't like being out of the water though, quite often when you lift rocks out they will let go back into the water, so I can't see them climbing out of the tank. I wouldn't worry I bet he is there somewhere it's amazing what little holes they wriggle into to the point that they look stuck. More than once I have intervened to remove a Hermit from what I thought was a space they were jammed in, only to find him back there 2 mins later. I don't bother now although my wife still sometimes says "I'm sure he is stuck" and I'm forced to act.