A B737-300 ON FIRST FLT OUT OF A C-CHK THE R PYLON R SIDE ACCESS PANEL WAS FOUND MISSING AFTER LNDG.

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JUL/THU/01 I WAS NOTIFIED THAT ACFT X AFTER LEAVING ZZZ ON JUL/THU/01 AT XA30 ARRIVED IN DCA AT APPROX XC30; WHERE IT WAS NOTICED THAT R ENG PYLON PANEL #5829R WAS MISSING. THE ACFT WAS RELEASED FROM C-CHK IN ZZZ ON JUL/WED/01 AT APPROX XA30. MY INVOLVEMENT WITH THIS ACFT WAS PARTIAL ACCOMPLISHMENT OF THE POST MAINT CHK CARD #XXXX-XXX; WHICH IS OUR FINAL INSPECTION CARD PRIOR TO RELEASE FROM C-CHK. I INSPECTED AREA OF MISSING PANEL JUL/WED/01 AT XA00 AND PANEL WAS DEFINITELY THERE AND APPEARED TO BE SECURED. NOTE: TIME OF INSPECTION WAS APPROX 37 HRS BEFORE ACFT WAS RELEASED DUE TO OTHER PROBS. I RECOMMEND A NEW POLICY BE IMPLEMENTED THAT THE POST MAINT CHK CARD MUST ONLY BE VALID IF THE ACFT IS RELEASED WITHIN 1 HR OF COMPLETION OR THE CARD SHOULD BE VOIDED AND REISSUED. ALSO; THE POST MAINT CARD PLAINLY STATES THAT IS ONLY A GENERAL VISUAL WALKAROUND. IN MY OPINION THIS SHOULD BE CHANGED TO A DETAILED INSPECTION. THE MISSING PANEL HAS CAM-LOCK TYPE FASTENERS. I RECOMMEND AN AIRWORTHINESS DIRECTIVE NOTE TO REQUIRE CLRLY MARKING AND PLACARDING INDEX MARKS TO INDICATE LATCHED AND UNLATCHED CONDITION THAT CAN BE SEEN FROM SEVERAL FT.