Dry drain trap

Trap water evaporates or is siphoned away, removing the sewer-gas barrier.

Typical fingerprint

Look for a repeatable relationship with location, weather, water use, equipment cycles or temperature. The source may be offset from the visible damage.

What strengthens this explanation

Multiple independent clues pointing to the same pathway are stronger than one photograph or meter value.

Repair principle

Control the source first, then address wet or damaged layers, verify drying or stability and finally restore finishes.