What was found
AIRBORNE RETURN DUE TO AIR CONTAMINATION OF AN UNKNOWN SUBSTANCE. NOTED SOOT ON BOTH SIDES OF FLIGHTR COMPARTMENT CAPTAIN AND FO SEATS ON FLAT AREAS NEAR TILLER AND WINDOW CONTROL. CLEANED UP SOOT AND R AND R'ED LOWER RECIRCUULATION AIR FILTERSREF AMM 21-25-08-4. CREW REPORTED NO SMELL OR FUMES. OPERATED APU AND ENGINES AFTER WARDS NO SOOT WAS NOTED. INSPECTED AIR DUCTS AT CONDITIONED AIR AND NO OIL OR CONTAMINANTS FOUND AT HOOK UP POINTS REF AMM 21-00-01-2. PERFORMED PACK CLEANOUT PROCEDURE REF AMM 21-00-01-2 NO SMELLS NOTED DUREING ALL OPERATIONS OK FOR SERVICE..