I will instruct my guards when checking a cell that appears empty to look for the chamber pot. If the chamber pot is still there, then the prisoner has escaped and they may enter and search for clues. If the chamber pot is not there, then either the prisoner is perched above the lintel waiting to strike them with it or else he decided to take it as a souvenir (in which case he is obviously deeply disturbed and poses no threat). Either way, there's no point in entering.