@Warmuffin: Probably because they put it only on the door, not accounting for the possibility that someone would go through the vents like Suya does. To be fair, though, how often would you find someone who'd willingly do that just for kicks? Suya wasn't even looking for a hidden room, she was just checking the bathroom of the other room and for some reason decided to explore the ventilation shaft.
Can wait for the misunderstandings when everyone finds out the princess sleeps in the demon monk's bed,or that he tells her not to come back but she does, and they also went house shopping
I just remembered that Suya isn't short. When they went to the human town, the demons were all really tall compared to the humans. So she should be normal height, right?