We've recently had our spare bedroom ceiling plastered, and there was a loosish bit of plasterboard that was secured by the plasterer before skimming the entire ceiling.
After painting it I observed a strange brown patch following the line of the underlying plasterboard. The only thing I have ever seen that is similar to this is the brown marks that appear on painted wood where a knotting solution hasn't been applied, or from the corrosion from underlying rusted nails.
The spot itself is directly below an attic water tank which looks like it may have leaked in the past (when the previous owners still lived here). However a thorough examination of the area revealed it to be bone dry (as was the area before plastering, and is now).
Does, anyone have any thoughts as to what might be causing this and steps I should take to remedy it?