Of course the first thought is that there could be something cool or creepy back there. Just so you know there may be a more mundane reason. Often plumbing and/or other structural features are covered with a facade from the outside and/or a wall panel inside. Just be prepared, who knows; the previous owner may have had an abrubt urge to wall in his or her spouse. 🙂 If you are handy and don’t mind having to repair any damage, you could just try popping a hole in and using a flashlight to check it out before making a huge hole.