This is why you have to introduce the concepts of mimics or demons that have access to change shape. Otherwise the party always frees the chained up maiden in the dungeon without asking any questions. Alternatively, if there is a rogue, you don’t have to worry. They’ll try their best to convince the others that they’ll get xp for stabbing the prisoner.
Okay, let’s make this simple enough for you. Pictures, everybody. Pictures for the man with the mind of a sloth. Exhibit A. Exhibit B.