On the first level of campaign theres a way out of the level into the large jungle behind it as well as up the cliffs where the pelican comes near the factory at the end of the level.
You need at least 2 players and you need to get to the factory roof, from there you can use a glitch to jump off your friends head till you reach a higher area, have your friend kill himself until he spawns on the clifface behind you and when he's there kill yourself, you can then freely wander around outside of most of the first map.
Credit to .Impact for the sig & avy