An upstairs bathroom leak
A bathroom is the most plumbing-dense small space in a house — a supply line to a sink, a toilet supply, a tub or shower valve behind a wall, and a drain assembly under a floor. Any of them can fail slowly. A valve that drips behind a shower wall, or a wax ring that's given up around a toilet, sends water down into the floor structure rather than onto the visible floor.
The consequence is that the bathroom itself may look fine — dry tile, no standing water — while the subfloor and the ceiling assembly below are both wet.
Water damage restoration