That is, until somebody spotted them on the Gears 3 leaderboards. The maps are titled "Azura", "Blood Drive" and "Rustlung". They're apparently not playable and are probably upcoming DLC.
Gears of War 3 was released in North America on September 20 to much fanfare. The game does have unlockable content, but like so many recent games, it also apparently has DLC waiting in the wings.
Kotaku is following up with Epic Games.