It was sweeper bot!

I don't think that there is any sort of traitor. The Cabal are a militaristic empire, and I find it hard to believe that they would engage an enemy without learning everything they can about them first. The Red Legion likely first made contact with the Cabal on Mars to receive any additional intel they didn't get in the numerous reports sent to them. I believe the quickly and easily found both the Traveler and the City below it on Earth, and set up an attack that was quick, efficient, and brutal. I think insinuating that the Cabal needed the help of an insider to pull this off is heavily underestimating them.