As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
AIR TURNBACK - DURING TAXING, WITH APU OFF, APU FIRE DETECTION INOP LIGHT ILLUMINATED. SHORTLY AFTER TAKEOFF APU FIRE WARNING CAME ON. APU FIRE HANDLE PULLED AND BOTTLES DISCHARGED. INSPECTED APU COMPARTMENT NO EVIDENCE OF FIRE. REMOVED AND REPLACED APU TAILPIPE FIRE LOOP DETECTOR PER MM.