As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
DURING FLIGHT AND RETURN TO BASE FROM FLIGHT, CREW NOTIFIED PILOT THAT THEY SMELLED A ELECTRICAL BURNING AROUND THE BACK CABIN WALL. IT WAS DARK INSIDE THE CABIN SO VISUAL IDENTIFICATION WAS DIFFICULT. PILOT AND CREW INSPECTED THE AREA BUT COULD NOT IDENTIFY THE SOURCE OF THE VAPOR.