After deactivating the final tower, run back to when you see the green Elite dropship. Quickly run to it and jump on, then kill the Elite on the turret.

Now this is a little tricky... run towards the turret while holding down RB and you will latch on to it. When it takes off to leave, you will leave with it. When it starts to disappear, you will look like you have Active Camo on but you're still on the turret!

Strange, huh? And the dropship doesn't seem to be there, but it is, and so are you.

Congratulations you have found another way to get out of The Covenant.