As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
"APU DET INOP" CAUTION LIGHT ILLUMINATED ON CLIMB-OUT AT 10000 FEET. IT CLEARED ITSELF ABOUT 5 MINUTES LATER. APU WAS NOT RUNNING. GROUNDED - YES. ACCOMPLISHED GVI OF APU UPPER, LOWER, AND TAILPIPE OVERHEAT DETECTORS AND WIRING CONNECTIONS - NO DEFECTS NOTED. PERFORMED TEST OF APU OVERHEAT DETECTION SYSTEM PER B737-800 AMM 26-10-00-710-801 AND ENGINE/APU FIRE DETECTION CONTROL MODULE TEST PER B737-800 FIM 26-10 TASK 801 - ALL TESTS PASS.