Damp: finding the source before painting over it
The stain shows up in one place and the source is almost always somewhere else. How to look, in order.
A damp patch does not show where the water gets in. It shows where it stopped. There can be six metres of slab between the two.
Start with the cheapest thing to check. Blocked gutters and downpipes account for a large share of façade damp, and are fixed in a morning. If the stain worsens in the first hours of heavy rain and dries quickly, that is almost always it.
Next, the joints. Window sealant and sill flashings last eight to ten years, and fail without anyone noticing. A stain that always appears below the same opening needs no further diagnosis.
If the stain is at ground floor level and rises from the base of the wall rather than running down it, this is not an infiltration, it is rising damp. The treatment is different and painting over it solves nothing: the paint lifts within three months.
Only after all this is opening up justified. A water tightness test costs a fraction of chasing out a wall to find a pipe that was fine.