FLC OF AN LGT DID NOT RECORD A FUSELAGE SCRAPE WHEN RPTING FUSELAGE CRACKS IN THE ACFT LOG DURING PREFLT.

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I RPTED TO WORK AND BEGAN TO PREFLT A B757. MY COMPANY; IN AN EFFORT TO SAVE COST; HAS STOPPED ROUTINE WALKAROUND INSPECTIONS BY ACFT MECHS AND RELIES ON PLT WALKAROUND INSPECTIONS TO IDENT DISCREPANCIES BEFORE AN ACFT IS DISPATCHED; AND MECHS TO WALKAROUND ON LAYOVER CHKS. DURING MY PREFLT OF SHIP I NOTED A BURNED OUT POS LIGHT WHICH HAD ALREADY BEEN MCO'D IN THE ACFT LOG. I ALSO NOTED A LARGE PATCH ON THE L SIDE FORWARD FUSELAGE AND ANOTHER LARGE PATCH UNDER THE AFT FUSELAGE AND A LONG ABRASIVE SCRAPE DOWN THE CTR OF THE AFT KEEL. THE SCRAPE WAS THE SAME COLOR OF THE SURROUNDING SKIN; IT HAD CONSIDERABLE OIL AND DIRT UNIFORMLY ALONG ITS ENTIRE LENGTH. THERE WAS NO SHINY METAL EXPOSED AND IT DID NOT GIVE THE APPEARANCE OF FRESHLY SCRAPED METAL. I LOOKED CAREFULLY TO DETERMINE THE EXTENT OF THE SCRAPE AND NOTED THAT THE LOWER VHF ANTENNA; JUST FORWARD OF THE SCRAPE; WAS UNDAMAGED. I CHKED TO SEE THAT A VISUAL LINE FROM THE SCRAPE TO THE BOTTOM OF THE MAIN LNDG GEAR INTERSECTED THE LOWER LIMIT OF THE ANTENNA. I SATISFIED MYSELF THAT ANY RECENT TAIL STRIKE WOULD HAVE DAMAGED THE ANTENNA WHICH WOULD THEN HAVE TO BE REPLACED. MY CONCLUSION WAS THAT IT WAS OLD DAMAGE AND I THOUGHT THAT THE AREA WOULD BE BURNISHED AND POLISHED DURING THE NEXT CALENDAR CHK. I SAW NO REASON TO ENTER MY OBSERVATION IN THE ACFT LOGBOOK BECAUSE JUDGING BY ITS APPEARANCE IT MUST SURELY HAVE BEEN OBSERVED BY MAINT PERSONNEL DURING A LAYOVER CHK. WE FLEW THE AIRPLANE FROM ATL TO STT TO STX AND BACK TO STT. THE NEXT 3 TKOFS AND LNDGS WERE NORMAL. NO UNUSUAL PITCH ATTITUDES WERE EXPERIENCED; NOR WAS ANYTHING UNUSUAL RPTED BY ANY PAX OR FLT ATTENDANT. IN ST THOMAS THE NEXT MORNING WITH ABOUT 1 HR ON THE GND I BECAME CURIOUS ABOUT THE LARGE PATCHES ON THE FORWARD AND AFT FUSELAGE. I INSPECTED THE FUSELAGE CAREFULLY TRYING TO SEE IF I COULD DETERMINE WHAT DAMAGE HAD REQUIRED SUCH CONSPICUOUS REPAIRS. I NOTICED ON THE AFT PATCH SEVERAL RANDOM CRACKS APPROX 1 INCH LONG AND OF MINIMUM WIDTH; BUT NOTICEABLE BECAUSE OF THE BLACK OILY APPEARANCE WHERE WATER; SOIL; AND OIL HAD PENETRATED. I WENT TO THE COCKPIT AND TOLD THE CAPT ABOUT THE CRACKS THAT I NOTICED. HE RETURNED WITH ME TO INSPECT THE AREA AND THEN TOLD ME TO ENTER A REQUEST TO HAVE THE CRACKS INSPECTED IN THE ACFT LOG. I AM TOLD THAT WHEN MAINT INSPECTED THE ACFT THEY WERE ALARMED; NOT BY THE CRACKS THAT I HAD RPTED; BUT BY THE SCRAPE THAT I HAD NOT RPTED. I BELIEVE THAT ROUTINE WALKAROUNDS BY QUALIFIED ACFT MECHS SHOULD BE REINSTATED.