All enemies in the game are permanently cloaked.
Progress through Delta Halo until you come to an area with a large central structure that has a multitude of columns (the place with the many stationary shield generators). This is the area where Cortana tells you that they are all pouring out of the middle.
When you first get into this area, you have to go left, and there is a ramp on the left that goes up and then back down. Get on this ramp and continue up the wall until you are on the landing where there will be (or used to be) some Plasma Turrets. Follow this landing all the way to its end, and you'll see a door that is blocked off by a large stone. Jump on top of the stone, and jump again to get on a thin ledge. Make sure you're up against the left wall, and Crouch Jump up to the top of the wall to your left.
Once you're on this wall, you'll see that the right wall ends and there is some earth behind it. Jump onto the dirt, and follow the stone wall all the way to the top. When you're on the top of this wall, you can look down towards the center and you should see two Stealth Elites repeating their "angry stance" towards each other with the Envy skull between them.