As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
PDX - ON FLIGHT NUMBER 2555, AFTER DEPARTURE THE AFT LAVATORY SMOKE DETECTOR WARNING MESSAGE APPEARED. FLIGHT ATTENDANT REPORTED SEEING SMOKE NEAR THE CEILING OF CABIN. SMOKE HAD AN ACRID SMELL TO IT. REPLACED THE 4TH STAGE SHUT OFF VALVE ON NR 2 ENGINE, FOR PRECAUTIONARY MEASURES. GROUND RAN ENGINES AND APU WITH NO SMOKE/SMELL NOTICED. AIRCRAFT RETURNED TO SERVICE.