A B737-300 FLC IN CRUISE AT FL350 RPTS A R ENG PYLON PANEL DEPARTED THE ACFT. PANEL WAS LOCATED ON TOP R PYLON JUST FORWARD OF THE WING LEADING EDGE.

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WE WERE STRAIGHT AND LEVEL AT FL350 WITH AUTOPLT B ON AND NO TURB. SHORTLY AFTER THE VOR; WE WERE NOTIFIED BY THE FLT ATTENDANTS THAT A 'PANEL HAD FLOWN OFF THE R ENG.' I HAD A JUMPSEATER ON BOARD SO I SENT HIM BACK TO LOOK. HE CONCURRED WITH THE FLT ATTENDANT ASSESSMENT. IMMEDIATELY CONTACTED MAINT CTL FOR GUIDANCE AS TO CONTINUE OR LAND. ZZZ1 AND ZZZ2 WERE BOTH VERY CLOSE OPTIONAL ARPTS. MAINT CTL; AFTER OUR BEST DESCRIPTION POSSIBLE; TOLD US WE COULD CONTINUE BASED ON THE KNOWN AREA OF THE PANEL AND THE FACT THAT NO ADVERSE HANDLING CHARACTERISTICS WERE EXPERIENCED; WE DID. THIS PANEL; ROUGHLY 7 X 9 IN SIZE AND LOCATED JUST FORWARD OF THE WING ON TOP OF THE ENG PYLON; APPEARED TO BE AN INSPECTION TYPE PANEL AND NOT A STRUCTURAL NOR SAFETY OF FLT ISSUE. ACFT TURNED OVER TO MAINT FOR REPAIR/DEFERRAL/CORRECTIVE MEASURES.CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: THE RPTR STATED THE AIRPLANE WAS IN CRUISE AT FL350 WITH AUTOPLT 'B' ENGAGED AND IN ABSOLUTELY SMOOTH AIR WHEN THE CABIN ATTENDANT RPTED A PAX HAD SEEN A PANEL DEPART THE AIRPLANE. THE RPTR SENT A DEADHEADING JUMPSEATER BACK TO INVESTIGATE AND HE VERIFIED A PANEL WAS MISSING ON THE TOP OF THE R PYLON JUST FORWARD OF THE WING LEADING EDGE. THE RPTR MADE THE WALKAROUND CHK PRIOR TO DEP AND NOTED NOTHING ABNORMAL; HOWEVER; THIS PANEL CANNOT BE SEEN FROM GND LEVEL ON A WALKAROUND INSPECTION.