AN SF340 RETURNED TO THE DEP ARPT DUE TO L ENG INBOARD COWLING DEPARTED THE ACFT. ON GND; FOUND COWLING MISSING AND GENERATOR STARTER BACKED OFF ACCESSORY GEAR BOX BY APPROX 6 INCHES.

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CHAIN OF EVENTS PRIOR TO ACFT RETURN TO FIELD. WAS ON THE LIST OF OIL CHKS TO DO. WE ARRIVED AT THE ACFT AT APPROX XA30. BEFORE COMPLETION OF THE CHK WE WERE CALLED FOR ANOTHER OIL CHK. WE FINISHED THE OIL CHK AND WENT ON TO THE NEXT ACFT. WE HAD BTWN 7-10 OIL CHKS NOT COUNTING OTHER WRITE-UPS. AFTER WE LEFT; IT SAT ON THE RAMP FOR APPROX 2 HRS BEFORE TKOF. APPROX XD30 WE WERE CALLED THAT ACFT WAS RETURNING TO GATE. WHEN WE CHKED THE ACFT WE DISCOVERED #1 ENG R-HAND INBOARD FORWARD COWLING WAS GONE. THE DC START/GENERATOR BAND WAS BROKEN AND THE START/GENERATOR WAS 6 INCHES BACK OUT OF ITS POS. THE COWLING LATCHES WERE FOUND LOCKED IN POS ON THE RECOVERED COWLING. AN INVESTIGATION BY MGMNT CONCLUDED THAT THE COWLINGS WERE NOT PINNED BUT LATCHED CLOSED. IT IS POSSIBLE THAT THE COWLINGS WERE NOT PINNED IN THE AIRFRAME PIN HOLE BUT THE LATCHES WERE CLOSED. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: AFTER THE L ENG COWLING WAS OPENED AND THE OIL CHKED; IT WAS CLOSED AND LATCHED. THE AIRPLANE WAS PARKED ON THE RAMP FOR 2 HRS AND THERE EXISTS A POSSIBILITY ANOTHER PERSON OPENED THE COWLING. THE COWLING WAS RECOVERED AND STILL HAD THE 2 LATCHES IN THE CLOSED AND LOCKED POS. ANOTHER POSSIBILITY WAS THE CONDITION THE STARTER GENERATOR WAS FOUND ON ARR. THE STARTER GENERATOR WAS APPROX 6 INCHES FROM THE ACCESSORY GEAR BOX MOUNTING CLAMP RING AND DEPARTED THE GEAR BOX WHILE TURNING AT HIGH SPD. THE FINDING OF THE STARTER GENERATOR DISCONNECT AT HIGH SPD BEING A POSSIBLE FACTOR WAS NEVER DISCUSSED.