What was found
AFTER CREW DEPLANED, GROUND CREW NOTICED FLAMES IN APU EXHAUST. FIRE CREW NOTIFIED. FLIGHT CREW ALERTED AND RETURNED TO AIRCRAFT. SMALL FLAME WAS OBSERVED IN THE APU EXHAUST WITH EVIDENCE OF FUEL ON THE RAMP BELOW THE APU. PWR FUEL SWITCH FOUND IN ON POSITION WITH EXTERNAL POWER ACTIVE. CREW SELECTED PWR FUEL SWITCH TO OFF. FLAME SELF EXTINGUISHED. MAINTENANCE INSPECTED. NO DAMAGE REPORTED BUT FUEL SMELL WAS PRESENT. APU DIFFERED PER MEL 49-10-11.