What was found
(CAN) ON DEPTR LAV SMOKE DETECTOR SOUNDED AROUND SAME TIME CREW STARTING FEELING NAUSEOUS. ALSO CREW RPTED PASSENGERS COMMENTING ON BURNING SMELL IN CABIN. A/C RETURNED TO YWG & LANDED W/O INCID.. MAINTENANCE INSP REVEALED APU OIL LEVEL DOWN TO 1/2 TANK - SUSPECTED SOURCE OF CONTAMINATION. ENGINEERS CARRIED OUT ENGINE RUN TO 80 PERCENT THRUST & PURGED AIR-COND DUCTS - SMELL DISSIPATED DURING RUNS. A/C CHECKED SERVICEABLE FOR POS ITION FLTPOSING FLT CREW RPTS SMOKE ODOR RETURNED, VERY EVIDENT IN COCKPIT ON DESCENT. A/C LANDED W/O INCID.. MAINT REPL APU, S/N: ON P37566, S/N: OFF P39229C - A/C GROUND CHECKED SERVICEABLE - NO FURTHER RPTS OF OIL SMELL IN CABIN OR COCKPIT.