Here is a prime example of value of performing a thermal imaging scan during the home inspection.  The bright blue circular area indicates the presence of moisture.Snow was melting on the roof and water was leaking down the bathroom vent duct to the bathroom ceiling vent (the dark red square). There was no visual staining on the ceiling. The moisture level measured in the drywall in the circular blue area was over 20%. In the attic a gleam of light reflected off  a stream of water was visible down the vent duct.