POSTFLT INSPECTION OF MLG REVEALED PANEL MISSING FROM ENG COWLING.

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FLT EWR-GSO. MAINT WAS WORKING ON ACFT BECAUSE OF FUEL LEAK PROBLEM. FO ACCOMPLISHED EXTERIOR PREFLT CHKLIST. I ACCOMPLISHED INTERIOR CHKLISTS WITH FO AFTER MAINT WAS FINISHED IN COCKPIT SAYING ELECTRICAL PWR COULD BE USED. BOARDED PAX. MAINT FINISHED WORK OUTSIDE ACFT. I HAD THE COPLT DO ANOTHER EXTERIOR INSPECTION WHICH HE DID. PUSHED FROM GATE. FLT WAS UNEVENTFUL. ARRIVED AT GSO. AS I WAS ACCOMPLISHING EXTERIOR POSTFLT; I NOTED A PANEL MISSING FROM #2 ENG STRUT (MOUNTING) OUTBOARD SIDE AS DID THE FUELER. SIZE OF MISSING PANEL APPROX 10 INCHES BY 18 INCHES. LOGBOOK WRITE UP MADE AND MAINT CONTACTED. SUBSEQUENT FLT WAS CANCELLED. SUBSEQUENT INVESTIGATION OF MAINT RECORDS INDICATED ON THE PREVIOUS NIGHT WORK WAS DONE ON THAT ENG. I CAN ONLY ASSUME THAT THE PANEL WAS THERE WHEN WE DEPARTED EWR GATE AS IT WOULD HAVE BEEN OBVIOUS IF MISSING TO FUELER OR DURING 1 OF THE 2 COPLT EXTERIOR INSPECTIONS. COPLT HAS 6 YRS AS COPLT OR CAPT ON THIS ACFT. IT MAY NOT HAVE BEEN PROPERLY SECURED. FROM MY POINT OF VIEW; I DON'T KNOW WHAT PLTS COULD HAVE DONE TO PREVENT THIS SITUATION OTHER THAN TO REEMPHASIZE TAKING SUFFICIENT TIME TO ACCOMPLISH THOROUGH INSPECTIONS (WHICH IN THIS CASE I FEEL WAS DONE).