This close to Halloween strange objects start to look like ghosts... Actually it probably is a reflection of some sort. It could be flare on the lens which reflects off the lens glass so that would be why you didn't see it on the wall when you took the shot - it was never there. Large windows and directional light from the fluorescent fixtures in the ceiling can cause this too. Doesn't happen often but when everything lines up just right, it can. A lens hood would help eliminate this.