As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
IN FLIGHT, APU FIRE WARNING SOUNDED. PERFORMED FIRE PROCEDURES AND ACTIVATED FIRE BOTTLES. AIRCRAFT RETURNED TO HOU AND MADE AN UNEVENTFUL LANDING. INSPECTED AND FOUND NO EVIDENCE OF FIRE OR OVERHEAT CONDITION. CONTINUED PER MEL. REMOVED AND REPLACED APU TAILPIPE FIRE DETECTOR PER BOEING M/M.