As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
BURNING SMELL NOTICED IN CABIN UPON TAKEOFF BY PILOT AND MED CREW. PILOT RETURN TO AIRPORT WHERE HE HAD JUST TAKEN OFF FROM. MAINTENANCE: PERFORMED GROUND RUN AND 20 MINUTE MAINTENANCE FLIGHT. NO CAUTION OR WARNING LIGHTS NOTED. OPERATED HEATER WITH NO ISSUES OR SMELLS NOTED. COULD NOT DUPLICATE BURNING SMELL. AIRCRAFT RETURNED TO SERVICE