Can't pinpoint source of leak

If this ‘leak’ is only showing a darkening of the paint , it may well be hygroscopic dampness. This is a condition where the plaster is so thick it absorbs moisture from the atmosphere and becomes wet that way . It’s very common and hardly anyone knows about it .

Its not plaster thickness particularly as formentioned but instead the type of plaster used that can be problematic. Such as modern non breathable/absorbent plasters incorrectly applied to a period or brick surfaces that are subsequently cold. If we are talking about a chimney breast/stack area? What may look like a leak can commonly instead be sulphate salts deposited in the masonry/plaster to the area that are hydroscopic. Causing condensation and damp patches. There are a number of very simple ways to confirm this that should have been ruled out/or in at the start - unfortunately. Too is the remedy to prevent this issue reoccurring relatively straightforward depending on how far you want to go. If you have had a roofer or damp 'specialist' assume the cause of your damp related issues and take money from you, for your issues to remain - that is appalling. But more common that not amongst the damp companies and building trade that don't fully understand what they profess to be experts in or exist to make a quick buck.