A DC6 CAPT REPORTS AN ENG ACCESSORY PANEL FELL OFF AFTER TKOF. THE FO AND SO FAILED TO SEE FASTENERS LOOSE DURING PREFLT.

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WE WERE TO SWAP ACFT DURING THE TURNAROUND. ON THE INBOUND LEG OF THE TRIP; I DELEGATED THE DUTIES UPON OUR ARR. THE SO WOULD GO TO THE NEW ACFT AND CONDUCT THE PREFLT INSPECTION; THE FO WOULD GO TO OPS AND DO THE PAPERWORK; AND I WOULD GO TO THE HANGAR AND EXCHANGE ACFT LOGBOOKS AND JOIN FO AT OPS. THE FO ASSERTED THAT HE WOULD PREFER TO GO TO THE ACFT AND ASSIST THE SO BY PREFLTING THE UPPER WING. THIS IS NOT SOP. BUT; IT WAS PAST MY BEDTIME. I DID NOT PROTEST. THE AREA WHERE THE ACFT WAS PARKED WAS VERY DARK. THE ACFT REQUIRED HEAT. AND; ENG BLANKETS MAY HAVE OBSCURED THE FASTENERS OF THE ACCESSORY PANELS. (I SAY 'MAY HAVE' BECAUSE THE FO AND THE SO CANNOT AGREE AS TO WHETHER BLANKETS WERE; OR WERE NOT; IN PLACE.) SHORTLY AFTER DEP; AFTER GEAR UP; I FELT A DISTURBANCE IN THE SEAT OF MY PANTS. IT WAS A SHORT YAWING MOTION. IT WAS SUBTLE. I ASKED THE CREW IF THEY HAD FELT IT OR SEEN ANYTHING ON THE PANEL. THEY HAD NOT. I PASSED IT OFF AS A DISTURBANCE CAUSED BY SOME BAD AIR. UPON ARR AT OUR ZZZ FACILITY; MAINT PERSONNEL BROUGHT TO MY ATTN THAT THE #1 ENG; UPPER AND LOWER; ACCESSORY COMPARTMENT ACCESS PANELS WERE MISSING. ABOUT 40% OF THE FASTENERS REMAINED IN THEIR RESPECTIVE SOCKETS. ACCORDING TO THE ACFT LOGBOOK; MAINT PERSONNEL MAY HAVE BEEN INSIDE THIS COMPARTMENT PRIOR TO OUR PREFLT. WHY A MECH WOULD PARTIALLY FASTEN A PANEL; IS UNKNOWN. WHY THE 2 AIRMEN I DELEGATED TO PREFLT THE ACFT DID NOT NOTICE THE PARTIALLY UNFASTENED PANELS; IS UNKNOWN. WHY THE GND CREW WHO REMOVED THE BLANKETS AND HEATERS FROM THE ENGS; DID NOT NOTICE THE PARTIALLY UNFASTENED PANELS; IS UNKNOWN.