This morning, the Microsoft press conference began with a live-action trailer for Halo 4, switching to in-game cinematics, and a gameplay demonstration. The trailer followed the exploits of Master Chief going after the downed USMC Infinity, running into a new enemy.
Cortana explains it as a defensive AI, not the Covenant. The trailer showed off two Forerunner weapons that dissolved the new enemies as well as a new visor that sees cloaked enemies.