As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
AIRCRAFT WAS NOT GROUNDED. AUXILIARY POWER UNIT DISCHARGE AGENT LIGHT ILLUMINATED INFLIGHT AND FL 380 WENT OFF IN DESCENT. PERFORMED FIRE DETECTION UNIT AUXILIARY POWER UNIT BUILT IN TEST EQUIPMENT TEST IN REFERENCE TO AMM 26-13-00. NO FAULT CODE NOTED AT THIS TIME.