They did, they even fought a battle over Antarctica.
This was covered up by having pretty much every orbital satellite turned off (my guess is by saying an unusually high solar storm, or something). They also fought over Antarctica in the same battle.
BUT, there are people who know “something” is going on, one of Cam’s friends (on SG-1 from Season 8/9-10), told Cam “Stop bullshitting me, I know you’re doing something – you didn’t crash in a test flight over Antarctica.”
People who are willing to look, it’s implied, know or at least have some idea that something’s going on. But the general public is blissfully ignorant by choice.