What was found
UPON DEPARTURE, SMELLED FUEL, THOUGHT POSSIBLY FROM VENT DUE TO ACFT BEING TOPPED OFF. UPON TURNING ON AIR CONDITIONER, MED CREW IN COPILOTS SEAT OBSERVED A GRAY MIST FROM THE COPILOT SIDE AIR VENT, INITIALLY PERCEIVED AS SMOKE, BUT WITH AN ENG COMP WASH ODOR. DUE TO THE FUEL ODOR & THEN GRAY MIST, ELECTED TO RETURN TO BASE, ENTERED DOWNWIND FOR LANDING & HAVE MECH CHECK OUT.