Hi. I have a strange problem. I have a random damp patch that has appeared on an internal wall out of nowhere. The patch is where a doorway has been pboarded up. Hallway is on the otherside of the wall. No pipes running under or behind. No visible dampness anywhere else. Have lived in house for 5/6 years without issue. More recently House has been sat empty for some time but i do use heating now and again so cannot explain it. Right hand side of pic is towards the exterior wall. Any ideas?