Damp in internal walls. Top floor flat

Jimbolahat

Hi
I live in a top floor flat which was newly built around 12 years ago. Over the last couple of months, the skirting boards on a couple of internal walls have started to bow away from the walls. Then in the last couple of weeks, small stains have appeared in a couple of places above the skirting.
There's no warping or staining to the engineered wood floor next to the skirting but there are radiators nearby. No evidence of water coming from above either. I'm assuming the walls are stud rather than solid.
On one internal wall it's happened on both sides. On the other wall where it's happened I can only see one side as the kitchen is fitted to the other side.
The pipes to the radiators seem to chase into the walls rather than go into the floor. There's no loss of pressure to the boiler.
I suppose my question is does anyone know how new flats are built in terms of pipework into internal walls and what happens if these pipes leak ? Does it result in warped skirting and staining ? Could it be condensation on the pipes as the heating has been off ? But where would the moist air come from ?
Could there be any other explaination for this before I remove the skirting and start slicing the wall up ?
Thanks in advance
Jimmy