We had a mark one inch above primed MDF skirting starting at corner of room (door end) and honestly thought it was a paint issue, we asked Dulux/painter and it baffled them. Now the mark has suddenly picked up momentum and travelled further along and stopped a metre away from the other end of the wall. We cut away the section of plasterboard and we thought it may have been a radiator leak as wall on other side is utility with radiator. Pipes dry, yet base of the timber the dwangs are fixed too is damp. Timber is dry at the end of the room towards outer wall direction (left as you look at picture). We are baffled as to the cause! Above this bedroom is our main bathroom so towel radiator and shower but there is absolutely no marks apart from the lower section of the wall. Would really, really appreciate any suggestions -help, would you speak to house insurer? Just don't know what to do. (House was built and finished 2013)