What was found
TWENTY MINUTES AFTER TAKEOFF AT 39,000 FEET, FLIGHT CREW REPORTED SMOKE IN CABIN & COCKPIT. CREW LANDED WITHOUT FURTHER INCIDENT. INSPECTED THE ACFT & PERFORMED GND OPERATIONAL CHECKS. COULD NOT DUPLICATE PROBLEM. FLIGHT CREW TOOK THE ACFT ON A FUNCTIONAL CHECK FLIGHT. THE FLIGHT WENT UP TO 35,000 FEET. DURATION OF THE FLIGHT WAS 1 & A HALF HOURS. NO SMOKE WAS DETECTED DURING FLIGHT. FUNCTIONAL CHECK FLIGHT WAS SATISFACTORY. ACFT WAS RETURNED TO SERVICE.