Moisture mapping revealed a wall cavity that could not drain at the opening.
Initial observation
The homeowner recorded the location, timing and a photograph before changing the surface.
Competing explanations
Several plausible causes were retained until a repeatable trigger separated them.
Decisive clue
The linked check produced a pattern that aligned with one cause and weakened the alternatives.
Resolution
The source was corrected before finish repair, affected material was allowed to stabilize and the original trigger was tested again.
Outcome
No recurrence was reported during the verification period. The timeline and evidence were retained as part of the house history.
Observe→Compare triggers→Repair source→Verify outcome
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