As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
SEA - LAVATORY SMOKE DETECTOR ALARM ACTIVATED IMMEDIATELY AFTER TAKEOFF. THERE WAS NO SMOKE OR HAZE IN CABIN. THE FLIGHT ATTENDANT PRESSED THE TEST BUTTON AND THE ALARM RESET. MAINTENANCE REPLACED THE LAVATORY SMOKE DETECTOR. OPERATIONAL CHECKS WERE GOOD AND THE AIRCRAFT WAS RETURNED TO SERVICE.