As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
#1 SOURCE OFF LIGHT ILLUMINATED AT TOP OF DESCENT. RAN QRH SOURCE OFF CHECKLIST. SOURCE OFF LIGHT EXTINGUISHED WHEN APU RUNNING AND ON BUS. GROUNDED - YES. ACCOMPLISHED B737-800 FIM 24-21 TASK 823. ALL CHECK NORMAL DETERMINED TO BE AN INTERMITTENT FAULT. CREW PERFORMED ENG RUN AND GROUND VERIFICATION WAS SUCCESSFUL. A/C OK FOR SVC. NO FAULT NOTED AT THIS TIME.