What was found
PRIOR TO THE FLIGHT, THE CREW WARMED UP THE ACFT WITH THE ENGS. THE CREW RAN THE SYS AT MAX TEMP SETTINGS. AFTER TAKEOFF, THE CABIN FILLED WITH A FAINT SMOKE. FLIGHT CREW RETURNED. COMPLIED WITH A SATISFACTORY ENVIRONMENTAL SYS INSPECTION & EXERCISED CABIN TEMP CONTROL VLV SYS & VERIFIED PROPER OPERATION. ACFT WAS TAKEN UP ON A FUNCTIONAL CHECK FLIGHT TO VERIFY DISCREPANCY. NO DEFECTS NOTED. ACFT RETURNED TO SERVICE. SUSPECT THE HEATED DUCTING ALONG WITH ANY RESIDUAL DUST INDUCED THEIR DISCREPANCY.