This never added up for me. It's clearly shown several times throughout SG-1 that the Goa'Uld have the technology to detect the exact location of a Stargate active or inactive anywhere on a planet's surface from orbit and their technology doesn't seem to have changed much in the thousands of years since Ra was on Earth and now, as seen in Moebius when SG1 travels back to Ancient Egypt and all Goa'Uld technology presented in that episode looks the same as it does during the rest of the show. So I can't believe that Ra didn't detect the Antarctic Stargate, or maybe he couldn't be bothered with it as it is a supposed inferior beta version of the Milky Way model.