As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
LEFT AFT LAVATORY SMOKE ALARM ACTIVATED AFTER TAKEOFF. FLIGHT WAS RETURNED TO DTW AND LANDED WITHOUT INCIDENT. PROBLEM COULD NOT BE DUPLICATED ON GROUND. IT IS SUSPECTED THAT RESIDUAL OIL FROM THE APU LEAKED INTO THE PNEUMATIC SYSTEM. AIRCRAFT WAS RETURNED TO SERVICE.