What was found
UPON DEPARTURE, FLIGHT CREW LEVELED OFF AT 5000 FEET MSL TO VERIFY PROPER FLAP OPERATION. IN LEVEL FLIGHT 160KTS OF AIRSPEED, RECEIVED "FLAP ASYMMETRY" INDICATION AFTER EXTENDING FLAPS 20 DEGREES TO 30 DEGREES. UPON TAXI IN, FLAP ASYM EXTINGUISHED AND FLAPS SET 30 DEGREES. / GAINED ACCESS TO FLAP CABLES AND BELL CRANKS. FOUND LEFT-HAND CABLES SLACK. UPON FURTHER INSPECTION FOUND FLAP CABLE SEPARATED FROM SWAG. NOTE: CABLE INTERNALLY CORRODED WITHIN SWAG. NO EXTERNAL VISUAL INDICATION PRIOR TO FAILURE.