As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
FIFTY TWO MINUTES INTO FLIGHT "MAIN CABIN SMOKE ALERT, LVL 3" ALERT AURAL WARNING ANNUNCIATED FOR 2-3 SECONDS. DONNED 02 MASKS BUT ALERT SILENCES IMMEDIATELY LAND NO SMOKE DETECTORS APPEARED ON AIR SYNOPTIC. PERFORMED OPS TEST ON SMOKE DETECTOR SYS IAW MM 26-14-01-5. NO DEFECTS NOTED.