A few months ago a water stain started to appear at the top of an internal wall in our new-to-us house (1920s semi). The yellowish stain can be seen on both sides of the wall, which is the hallway and a box room. No sign of it on the ceilings. It doesn't feel wet and not particularly damp either, mostly just coldish (which I suppose is actually damp). There is wallpaper in the box room which is coming off the wall due to the moisture. Looks almost like big bubbles. Sometimes they seem to dry out and 'deflate' (?).
There is a loft conversion above it and some boxed in pipes right next to the stain in the box room. We've had the plumber come round twice. He checked the central heating but it's not losing pressure so not the culprit. He opened the boxed in pipes right next to the stain but everything was dry. He spent ages digging around the loft eaves directly above where the pipes run but couldn't find anything, all dry. So then we had a roofer come and he couldn't find anything in the area of the stain but we had him fix all broken slates just in case. But it's still slowly growing and darkening. Box room is the baby's room and so I really want this to be solved!
I have been documenting when we use the loft bathroom, when it rains, and when the wallpaper 'bubbles' are dry/deflated and when they seem damp/puffed up. I can't find any correlation. The plumber suggests we might want to cut into the ceiling to investigate but I really want to avoid that. Especially because there is absolutely no sign of moisture on the actual ceiling.
Anyone have any ideas? Other things to investigate or check? Could it just be condensation? But then, would that be visible on both sides of the wall? RH tends to be 50-60% at 18°C.
I've attached a picture. Left is the hallway side and right is the box room bubbling wallpaper.