As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
CREW NOTICED HAZY SMOKE AND HEAT SOURCE BETWEEN ROWS 4 AND 10 ON AIRCRAFT LEFT SIDE LIGHTING VALANCE. MX PERFORMED APU CHECKS WITH BLEED PACKS IN ALL CONFIGURATIONS AND ENGINE RUNS WITH APU OFF AND BLEED/PACKS IN ALL CONFIGURATIONS AND FOUND NO DEFECTS. MX COULD NOT DUPLICATE THE PROBLEM AND AIRCRAFT WAS SIGNED OFF FOR SERVICE.