As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
GROUND CREW REPORTED SMOKE IN AFT PIT. SMOKE FROM DUCTING AND FORWARD WALL AREA. CAUSE APPEARS TO BE A STUCK AFT CARGO HEATER RELAY. STUCK RELAY ALLOWED HEATER TO OVERHEAT TO POINT WHERE SILICONE WRAPPED HEATER RIBBON MELTED, CAUSING SMOKE. MELTING OF RIBBON WIRE CAUSED OPEN CIRCUIT, ALLOWING HEATER TO SHUT DOWN. RELAY K10083 PER WDM 21-44-11. RELAY, HEATER AND FAN REPLACED.