As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
AIR TURNBACK, DUE TO RED APU FIRE MASTER WARNING INDICATION. CREW FOLLOWED CRH PROCEDURES AND DECLARED AN EMERGENCY. AIRCRAFT LANDED WITHOUT INCIDENT. REPLACED THE RT FIRE EXTINGUISHED CONTAINER, APU SQUIB, AND APU FIRE LOOPS. OPS CHECK GOOD.