A B737-800 WAS RETURNED TO SVC AND OPERATED FOR SEVERAL DAYS WITH THE STANDBY HYDRAULIC SYSTEM FOUND PINNED IN THE 'OFF' POSITION.

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I WAS INFORMED THAT AFTER MY FLTS ON THIS AIRPLANE THE STANDBY HYD SYSTEM WAS FOUND PINNED IN THE OFF POSITION. I PREFLTED THIS AIRPLANE. I ALWAYS DO A THOROUGH EXTERIOR PREFLT. MY PHILOSOPHY IS THAT I WANT TO FIND PROBLEMS ON THE GND; NOT AIRBORNE. THE UNION INFORMED ME THAT THE STANDBY HYDRAULIC SYSTEM WAS FOUND PINNED OFF AND THE PIN HAD A FLAG ON IT. I GUARANTEE THE FLAG WAS NOT VISIBLE ON ANY OF MY PREFLTS. THE PIN MAY HAVE BEEN IN PLACE AND THE FLAG TUCKED OUT OF VIEW. I HAVE A LOT OF TIME ON THE AIRPLANE AND KNOW IT WELL BUT AM UNAWARE OF THIS MAINT PROC AND ITS PITFALLS. I WAS ALSO UNAWARE/UNTRAINED THAT THE STANDBY SYSTEM COULD EVEN BE PINNED OFF. IT IS DIFFICULT TO COME UP WITH A RECOMMENDATION ON HOW TO PREVENT THIS KIND OF THING IN THE FUTURE. IT IS IMPOSSIBLE TO COME UP WITH A METHOD BY WHICH A PLT CAN CATCH EVERY POSSIBLE MAINT FAILURE. SURELY THERE ARE PROCS IN PLACE TO CHK AND DOUBLE CHK WORK? THERE WERE REDUNDANT CHK PROCS IN PLACE DURING MY TIME IN MIL AVIATION MAINT WORK. FROM A PLT PERSPECTIVE; THE BEST RECOMMENDATION THAT I CAN SUGGEST IS THAT ALL MAINT PINS SHOULD HAVE FOOLPROOF VISIBILITY DEVICE ATTACHED TO IT THAT CANNOT RENDER ITSELF INDISCERNIBLE FROM A NORMAL ACFT APPEARANCE.CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: THE RPTR STATED THE FLAG MUST HAVE BEEN ROLLED UP AND HIDDEN BEHIND THE HYDRAULIC COMPONENTS IN THE FWD WALL OF THE MAIN WHEEL WELL. THERE IS NO COCKPIT TEST THAT WOULD REVEAL THE BLOCKED HYDRAULIC SYSTEM. ALL OF THE WALK AROUND INSPECTIONS WERE PERFORMED IN A PROFESSIONAL MANNER.