What was found
(CAN) 1.5 HOURS AFTER LANDING, SMOKE WAS SEEN COMING FROM THE AUXILIARY POWER UNIT. THE APU WAS SELECTED OFF FROM THE COCKPIT. INSPECTION FOUND FUEL IN THE APU OIL TANK AND EVIDENCE OF BURNING AT THE EXHAUST MUFFLER. FURTHER INSPECTION FOUND A FCU BOLT HAD BACKED OFF. THE FCU WAS REPLACED AND THE FILTERS WERE REPLACED. THE APU WAS GROUND RUN SERVICEABLE.