Fuel Leak

[JamesC]

OK here is a problem I haven't seen yet. Had a great trip to the Bahamas and back with no squawks except when I refuelled at KFLO, minor fuel stains developed over night on the under side of BOTH wing roots. The tank overflow drains were not dripping and the filler neck drains were clear. Underwing panels inboard of sumps were removed and nothing obvious found. Nil if any stains on the ground. The stains were cleaned off the fuselage and after landing at KSYR and having the acft sit over night, no fuel yet added, had no stains. However they began developing after the top off the next day. Minor fuel smell in cabin when you first opened it up. Fuel selectors were never changed from green/green (1979 P337H). My mechanic is going to have a look but appears a bit stumped at this point.
Has anyone had this ?