The back room has a bed with a bare mattress, a wardrobe with nothing in it, and dust undisturbed for years.

Except the dust on the windowsill has been wiped. Recently. In the shape of a hand.

You open the window. The room smells like a person and there is nobody in it.

That night the bed is made when you check it again, and the wardrobe door is open.

You put a lamp in there and leave it on.