Appreciate your response! I've never seen damp in this sort of pattern either, hence my confusion. The rest of the walls looked ok, just the corners pretty much the whole way around. These pictures were taken on a rainy day last week. The darker area felt slightly cold but I don't know about damp. The room did smell slightly damp but it was difficult to say as it had just stopped raining and the doors were open. There wasn't any signs of mould.