What was found
AFTER THE NORMAL REFUELING, REMOVED HATCH AND FUELED FROM THE TOP OF AIRPLANE, WENT TOPSIDE TO INSPECT THE FUEL CAPS. ON RETURNING TO THE CABIN, REINSTALLED THE HATCH. ROTATED THE HANDLE TO THE "LOCKED" POSITION, THERE WAS A POP AND THE HANDLE CAME OFF IN HAND. NOTICED THE ROLL PIN THAT RETAINS THE HANDLE TO THE SHAFT OF THE LOCKING MECHANISM WAS LYING ON THE FLOOR BELOW. REINSTALLED HANDLE TO SHAFT AND REINSERTED THE ROLL PIN TO RETAIN THE HANDLE. HATCH APPEARED SECURE. CRANKED AND TAXIED FOR TAKEOFF. TAKEOFF NORMAL. DIVERTED ATTENTION TO OVERHEAD PANEL TO MOVE SWITCHES AND NOTICED HATCH GONE. LANDED AND INSPECTED HATCH AND COULD NOT FIND ANY DAMAGE TO HATCH.