Initial observation
A small mark appeared during storms but stayed unchanged during vertical rain.
The misleading theory
A nearby bathroom made plumbing the first suspicion, but water-use testing produced no change.
The clue that changed the path
Both events occurred during southwest wind.
Resolved outcome
A flashing return was corrected, affected layers dried and no recurrence appeared.
Observe→Compare triggers→Repair source→Verify outcome
Living field guideThis useful article is public now. Sources, cases and graph connections continue improving without blocking publication.