A B737-700 DURING AN FAA RAMP INSPECTION THE INSPECTOR DISCOVERED A CRACKED LENS ON THE L WINGTIP LIGHT NOT FOUND BY THE FO ON THE WALKAROUND CHK.

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I ARRIVED AT THE ACFT AND WALKED INTO THE COCKPIT. AN FAA INSPECTOR PLOPPED DOWN ON THE JUMPSEAT AND INFORMED ME HE NEEDED TO SEE MY LICENSE AND MEDICAL. (HE HAD NOT YET SPOKEN TO CAPT XXX). I TOLD HIM IT WAS IN MY SUITCASE AND I WOULD GET IT IN A COUPLE OF MINS AFTER GETTING THE APU GOING. HE GRABBED THE LOGBOOK AND EXITED THE COCKPIT. I PRESENTED MY MEDICAL AND LICENSE AND THEN DID THE WALKAROUND. THE INSPECTOR AND CAPT MET ME AT THE BOTTOM OF THE STAIRS AND THE INSPECTOR SAID HE WANTED TO SHOW ME SOMETHING. HE STATED HE HAD ALREADY DONE A WALKAROUND BEFORE WE ARRIVED (APPROX 1 HR BEFORE). HE POINTED OUT A CRACKED LENS COVER ON THE L WINGTIP (WHITE POS LIGHT) AND A SCRATCH ON THE TIP. WITH THE POS LIGHT ON (WHICH IT WAS NOT WHEN HE WALKED AROUND) IT WAS VERY DIFFICULT TO SEE THAT THE COVER WAS CRACKED. CONTRACT MAINT WAS CONTACTED AND AFTER WAITING FOR AWHILE; WE DID AN ACFT SWAP. WE PUSHED 1 HR 25 MINS LATE. WHILE DOING THE WORKAROUND ON THE NEW ACFT; THE INSPECTOR APCHED ME AND INFORMED ME THAT HE MAY HAVE TO FILE AN ADMINISTRATIVE ACTION AGAINST ME FOR NOT DOING A PROPER WALKAROUND/PREFLT. PROPER PROCS WERE FOLLOWED ON THE WALKAROUND AND PREFLT. I SIMPLY MISSED SOMETHING THAT WAS DIFFICULT TO SEE. THIS HAD PROBABLY BEEN MISSED BY SEVERAL FO'S. HAD THE INSPECTOR INFORMED OUR ACR OF THE PROB WHEN HE NOTICED IT; THE FLT MAY HAVE PUSHED ON TIME OR ONLY A FEW MINS LATE.