The space between Siriani's hotel room and Garnier's hotel room.
Frank Siriani is supposed to be able to watch Mme. Garnier thru a hole in the wall that he discovered when the Crow keeps falling down , however? External shots of the hallway show that there is a door between the rooms (possible a cleaning closet) that is approximately 2.5-3 feet wide which means that the "peephole" Siriana is using would only look into either the closet or a deadspace.