What was found
BIL - FLIGHT 2481 - CREW RETURNED TO FIELD WHEN AFTER TAKE-OFF THE AFT LAVATORY SMOKE DETECTOR SOUNDED. THE CREW COULD SMELL OIL BUT THE ODOR SOON DISSIPATED AND THE WARNING WENT OUT. MAINTENANCE INSPECTED THE AIRCRAFT AND FOUND OIL IN THE MIXING MANIFOLD. MAINTENANCE VERIFIED THAT THE APU WAS LEAKING OIL AND THE APU WAS MEL'D. THE DUCTING WAS CLEANED AND OPERATIONAL CHECKS WERE GOOD. THE AIRCRAFT WAS RETURNED TO SERVICE.