Chasing the leak behind the wall

The stain is in the unit below; the leak is two floors up in a wall nobody wants to open. This thread is for moisture detective work: meters, borescopes, and the order of operations that finds the source before you close the ceiling. Tell us your best hidden-leak story and what finally gave it away.

Water is a liar: it enters in one place, travels along whatever path the building offers, and introduces itself two floors down and ten feet sideways. The techs who are good at this work in order — meter first, then the wettest point, then upstream — and they open the smallest hole that answers the question. Closing a ceiling over an unproven fix is just scheduling the second opening. What's the furthest a leak's source ever was from its stain on one of your properties — and what finally gave it away?