As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
AIRCRAFT RETURNED TO THE FIELD AT SFO UNDER AMBER ALERT DUE TO A FIRE BELL AURAL WARNING. *S/D* THE ENGINE FIRE DETECTION SYSTEM CAUSED THE AURAL WARNING. A BITE TEST WAS PERFORMED AND INDICATED NO FAULTS. THE M1000 BELL CHIME MODULE WAS REPLACED AS A PRECAUTION. THIS MODULE WAS DETERMINED TO BE SERVICEABLE AND WAS RETURNED TO SERVICE. NO OTHER FAULTS HAVE BEEN INDICATED SINCE THIS INCIDENT.