I thought I was the only one who experienced something seemingly impossible as this. For me, it was an african cichlid of some sort. I remember finding it pretty dried out. I put it in the tank, saw it move just a little after I moved it in the water, and left for school. That evening it was fine. I was about 14 years old and I am 100% sure my dad didn't replace it.
This prompted me to google, this is all I found:
It's one of those worst moments when you discover a fish missing from your tank — a rudimentary look revealing the animal in question, board-stiff and dessicated on the floor. Here's what to do...
www.practicalfishkeeping.co.uk
I wonder how long my fish had been out, I have no way to know. It DEFINITELY looked dead and dried out big time/stiff.