Discovery
Airline ground personnel identified heat and fire damage in the aft cargo bay, specifically in the vicinity of the mid-ship drain mast.
Investigation
The exact timing of the damage could not be determined. There was no recorded crew report of a fire or smoke detection warning. A functional check of the relevant systems confirmed they were capable of normal operation. No circuit breakers were found in the popped position. The drain mast and its associated ground/air relay switch were tested at the manufacturer's facility, and no evidence of failure was found in those components.
Background
Previous similar occurrences have been attributed to inadequate housekeeping. The area around the drain mast is a low point in the fuselage, making it a natural location for trash and litter to accumulate.