I always thought it was building the blue bedroom that triggered smelling the room. It may, I'll have to check that out, but with my current family I built the second bath before either the blue bedroom or the "small central room" which is the middle room at the back of the house (not the room dead center, although that isn't built also). So either the behavior is triggered by the bathroom, or by building a certain number of rooms, or by building rooms in that part of the house, or it has multiple triggers. I'm really hoping that it's a puzzle that we have yet to solve, rather that just a random thing.