What was found
AFTER TAKE OFF CREW REPORTED THE LAVATORY SMOKE WARNING CAME ON FOR 5 MINUTES. FLIGHT ATTENDANT ALSO REPORTED AN MIST IN THE AIR WITH A BURNING SMELL. AN EMERGENCY WAS DECLARED AND AIRCRAFT RETURNED TO FIELD WITH NO FURTHER INCIDENTS REPORTED. CONTRACT MAINTENANCE ISOLATED SMELL TO APU. MAINTENANCE FOUND A LOOSE OIL DRAIN LINE WAS TIGHTENED AND AN OPERATIONAL CHECK WAS COMPLIED WITH. AIRCRAFT WAS THEN RETURNED TO SERVICE.