What is causing a wet patch on an internal wall?
It will be awfully hard to say what is causing the damp spot without either opening up the wall or knowing a lot about the context in terms of what is in and around the area.  Either way, if the spot is damp and remains damp, repair will involve removing the wet section, so I'd say the answer you're looking for is: open up the wall by removing the wet spot, then look around and above the wet spot for potential sources.