What was found
F/A REPORTED SMOKE BUILDING IN THE AFT CABIN ON FINAL APPROACH ACCOMPANIED BY THE AFT RT LAVATORY SMOKE DETECTOR IN ALARM. VISIBLE SMOKE WAS OBSERVED INSIDE LAVATORY SO THE F/A DISCHARGED THE PORTABLE FIRE EXTINGUISHER INTO THE AFT LAVATORY AND SECURED THE DOOR. MAINTENANCE FOUND A GASPER DUCT BEHIND THE AFT LAVATORY MIRROR DISCONNECTED AND EMITTING MIST. THE DUCT WAS SECURED AND A PNEUMATIC SYS BURN-OUT WAS PERFORMED. NO FURTHER VISIBLE SMOKE AND NO ODORS NOTED SO THE ACFT HAS RETUNED TO SERVICE.