There was peeling and bubbling paint in the area (bottom right) where the brownish stains are in the picture. 
That wall has been scraped, sanded and skim coated. I think the wall is plaster.
What would cause that brown stain?
There is plumbing behind that wall. The shower head is in the other side of the wall ( a foot or so above that stain and maybe a foot to the left)
Best Answer
Moisture. Given the sequence of events resulting in the appearance of the brown spots and an obvious source of moisture, from the piping, one can rule out "grease", "tar", "solvents", "oils", and etc. Whatever caused the previous paint to "bubble and peel" is continuing to impact this area.