As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
LAV SMOKE WARNING MESSAGE DURING DESCENT. (FLT#3778) CLOSED REMOVED AND REPLACED AFT LAV SMOKE DETECTOR IAW AMM. AFT LAV SMOKE TESTS OK ON GROUND. PERFORMED OPS CHECK OF LAV SMOKE WITH APU AND ENGINE BLEEDS. OPS CHECK GOOD. CREW NOTED THAT THERE WAS A PAX IN THE LAV AT THE TIME OF SMOKE WARNING BUT WERE UNABLE TO CONFIRM SMOKING OR VAPING IN LAV. NO SMELLS WERE NOTED.