As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
SMOKE DETECTOR IN AFT RIGHT LAVATORY ACTIVATED AFTER TAKEOFF. FLIGHT RETURNED TO DTW AND LANDED WITHOUT INCIDENT. MAINTENANCE FOUND APU OIL TEST PORT LINE B-NUT LOOSE. MAINTENANCE TIGHTENED NUT AND REPLACED BOTH WATER SEPARATOR BAGS AND SOCKS, OPERATIONAL CHECK NORMAL.