What was found
A FUEL ODOR BECAME NOTICEABLE THEN STRONG IN THE CABIN OVER A SHORT PERIOD OF TIME. THE WHOLE FUEL SYSTEM WAS CHECKED AND NO LEAKS WERE VISIBLE. ONLY AFTER A FUEL STAIN (RED MOTOR GAS) WAS SEEN ON THE BELLY WAS THE SEEP TRACED TO THE FUEL (STRAINER SUMP) WHERE THE O-RING SEAL AT THE TOP HAD DETERIORATED ALLOWING AN ALMOST INVISIBLE SEEP. O-RING REPLACMENT STOPPED THE PROBLEM. TIME AT 3,600 HOURS.