As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
(B18710) AIR TURN BACK, DUE TO MAIN DECK FIRE WARNING. NO FIRE EVIDENCE WAS FOUND AFTER ACFT TURNED BACK. CHECKED ELECTRONIC COMPARTMENT FOR ANY LEAKS AND THE CONDITION OF THE SEALANT OF THE FLOOR PANELS. THE RESULT IS NORMAL. WATER RESIDUAL AND PAPER TOWELS WERE FOUND AROUND 1L DOOR. THIS MIGHT CAUSE THE "FIRE MAIN DECK" ILLUMINATED DUE TO THE SMOKE DETECTOR DETECTED THE VAPOR. NO MORE FALSE WARNING FOR SIX MONTHS.