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 DURING CLIMB. SMELL WAS NOTICED FROM THE FLIGHT DECK, FA REPORTED NO SIGN OF SMOKE IN THE LAV. RETURNED TO FIELD. (FLT#3291) FO: 03/06/2019 CLOSED REMOVED AND REPLACED AFT LAV SMOKE DETECTOR IAW AMM. OPS CHECK GOOD AT THIS TIME.