While playing Nightfall I noticed a closed gate in the base just before meeting the Guda (forgot how to spell it). So I spent about 30mins or so gathering up all of the forklifts, I think there is 3 or four and piled them into the corner (as seen on attach map). Each FL push the next FL a little bit higher, to the point where I was able to Jump over the Gate. Unfortunately nothing was found but it game me an idea.
While you guys are playing see if you notice inaccessible areas or heights not normally reach by normal means. Vehicles and weapons are key for single player use but playing 4 player co-op is the ultimate ladder.
Feel free to see if you can jump the gate, you may even be able to REACH a new spot if you are playing co-op.