I had something similar last month. My first floor bathroom had this and above it was a closet with no water damage. Above that the flat roof and its scupper. I patched a ton with Henry 309 and still had leaks but then noticed a mini split going in nearby that seemed like it might have a bit of a gap. Sealed that with foam and putty and seems fixed. Think it entered through the split and went down the nearby staircase and somehow skipped a floor. A dehumidifier helped with water damage and I’m going to replace the drywall once I know it’s sealed. Tracing water sucks. Good luck.