When Peter leaves the motel room to dispose of the smoke alarm, Agnes uses the bathroom with the door open and we see that the toilet is directly next to the open door. In later scenes, the sink is in the position directly next to the bathroom door.
Throughout all the scenes in the motel where the bathroom is visible, we see that the bathroom door is a regular standard door. But in the scene when Peter is removing a tooth, as he goes to enter the bathroom the door changes into a double louvered door. Then changes back to the regular door in later scenes.
Near the end, when a pizza is delivered to Agnes and Peter, she takes some money and goes to the door : one can see she has a panties.
A few minutes later, when they undress themselves, Agnes just removes her nightie : her panties has disappeared already (and she never removed it before that last scene).