During Miss de la Rue's number her dressing room's door was opened.

It was very careless of you to leave your door ajar.

On the landing he was surprised to see that the stranger's door was ajar.

However, once the cleaning woman had left the door to the living room slightly ajar, and it remained open even when the lodgers came in in the evening and the lights were put on.

The door to the hall was ajar, and since the door to the apartment was also open, one could see out into the landing of the apartment and the start of the staircase going down.