As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
APU FIRE WARNING SWITCH ILLUMINATED IN FLIGHT. APU WAS NOT RUNNING. FIRE HANDLE WAS PULLED, STILL ILLUMINATED. DISCHARGED FIRE BOTTLE, LIGHT EXTINGUISHED. INSPECTED APU AREA FOR FIRE DAMAGE, NONE NOTED. PULLED AND COLLARD APU FIRE DETECTION CIRCUIT BREAKER AND PLACED ITEM ON MEL FURTHER TROUBLESHOOTING REQUIRED. REMOVED AND REPLACED APU TAILPIPE FIRE DETECTOR LOOP IAW B737 MM 26-15.