I was going to bite my tongue before commenting on this, but that is the answer. When each prisoner goes into the room they will cut themselves and leave their

I was going to bite my tongue before commenting on this, but that is the answer. When each prisoner goes into the room they will cut themselves and leave their mark in blood in the room. Or they could scratch the wall etc. Before you say this won't work because the warden could mess with the marks I don't have 30 something years to wait around for randomness to finish.