A B737-300 FIRST FLT OUT OF A MAINT CHK HAD A PYLON PANEL DEPART THE ACFT. ALL PYLON PANELS WERE CHKED PER CHK JOB CARD FOR LOOSE OR MISSING FASTENERS AND ALL FASTENERS INDICATED LOCKED.

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ON AUG/WED/06; ACFT WAS SCHEDULED FOR A MAINT VISIT CHK IN THE HANGAR. MY APPRENTICE LEAD ASSIGNED ME TO THE MAINT CHK THAT NIGHT. I DO RECALL IN THE MAINT CHK PAPERWORK THERE WERE FIGURES ON THE FASTENERS OR SCREWS IN QUESTION TO BE INSPECTED AND TIGHTENED. I ACCOMPLISHED THIS INSPECTION BY GOING ON TOP OF THE L AND R WING TO ACCESS THESE SCREWS PER MY MAINT CHK PAPERWORK. THOUGH I AM AN APPRENTICE TECHNICIAN I AM TRAINED ON THESE SCREWS AS WELL AS THE PYLON PANEL FASTENERS. I UNDERSTAND ALSO THAT THE CREWS AND PANELS ARE SUBJECT TO LOTS OF VIBRATION DUE TO THE FACT THAT THEY ARE RIGHT ABOVE A RUNNING ENG. MY FINDINGS WERE CONCLUSIVE AT THE TIME; THAT NO FASTENERS OR SCREWS FOR THAT MATTER WERE MISSING OR LOOSE. IF I HAD FOUND ANY DISCREPANCY WITH THE SCREWS OR PYLON PANELS I WOULD HAVE DONE WHAT I WAS TRAINED TO DO. THAT IS; GENERATE A NON-ROUTINE CARD TO DOCUMENT THE DISCREPANCY ON MY FINDINGS. ALSO; I WOULD HAVE DOCUMENTED THE ACTION I TOOK TO FIX THE LOOSE SCREWS OR PYLON PANELS ON THE SAME NRC. MY BELIEF IN THE MATTER IS THAT DUE TO THE INTENSE VIBRATION THE SCREWS GO THROUGH DAY IN DAY OUT; SOME GET LOOSE FROM THE NUT PLATE AND COULD COME OUT AND GO MISSING.