What was found
LEFT DUCT LEAK CAME ON SHORTLY AFTER TAKEOFF. SHUT DOWN NR 1 ENGINE AND THE LIGHT WENT OUT. RETURNED TO SAN. REMOVED INBOARD LEADING EDGE AND WING ROOT FAIRINGS (LEFT). INSPECTED ALL LINES, DUCTS AND CLAMPS, NO LEAKS NOTED. OHMED OUT ALL DUCT SENSORS, NO DEFECTS NOTED. CHECKED WATER SEPARATOR AND 18 PSI REGULATOR, NO LEAKS NOTED. COULD NOT DUPLICATE PROBLEM. OPS CHECKS GOOD.