Definitely a moisture problem with your wall. If I understand this correctly, the wall is between your bathroom and living room. This is an interior wall.
It is weird to have this problem on an interior wall. You have already check the two major areas, plumbing and roof. Do you have a crawl space under your house? If so, this is a good area to look at. Hopefully the problem will be more evident from a different angle.
I do have one suggestion;
Go ahead and scrape or strip off as much of the paint as possible. Let the wall breath for a couple of days. Then apply a piece of 6 mil plastic, 2ftx2ft, with duck tape to the wall. Tape the edges solid. Let the plastic sit on the wall for 2 days then peel it off and see if any water has condensed on the wall or plastic.
If so, then there is water in the wall. The source of the moisture should be found before repainting. If this proves too difficult, go ahead and repaint with a flat paint. Keep trying to find the source of the water. Left unfixed this could cause rot and mold damage.