a few more tricks:
tape a string on the top door frame. if it open, then close again,the string will be stuck in the middle of the door and the frame if you know what i mean. make sure you have the correct string's length though.

some flour on the ground, the door open = wind = not a pile anymore.

put a book overlap your stuffs in the drawer or something, if they want something from there, the book got to move. then you would notice.

or a marble in a corner. once you open it, it will move :P

if there IS someone, then I'm doubting your dog's abilities. is she just a pet? or the house keeping type?