What was found
IN CRUISE VIA AIRLINK, CREW REPORTS POSSIBLE FUEL LEAK FROM LEFT MAIN TANK, UNABLE TO COMPENSATE, FUEL LEAK IS NOT INTERNAL, 2900LBS FUEL IN-BALANCE, UNABLE TO CONTROL WITH CROSSFEED. AS A PRECAUTIONARY MEASURE, AIR TURN BACK TO KPHX WITH AN EMERGENCY DECLARATION TO ATC. AIRCRAFT GROUNDED FOR UNSCHEDULED MAINTENANCE. DETAILED VISUAL INSPECTION OF SYSTEM IN ADDITION TO MULTIPLE FUEL TRANSFER CHECKS SATISFACTORY WITH NO LEAKS NOTED. ENGINE RUNS WITH FUEL FLOW AND FEED NORMAL. FAULT NOT DUPLICATED. NO DEFECTS NOTED. AIRCRAFT RELEASED TO SERVICE.