As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
SMOKE AFT LAV MASTER WARNING MESSAGE FOR SEVERAL SECONDS AFTER TAKEOFF. NO SIGN OR SMELL OF ACTUAL SMOKE, MESSAGE EXTINGUISHED ON ITS OWN. (FLT 5540) RAN BOTH AIR CONDITIONING PACKS AND FOUND NO DEFECTS. PERFORMED OPS CHECK OF THE LAVATORY SMOKE DETECTOR AND FOUND NO FAULT.