Was jumping backwards away from enemies when I realized I was on a ledge. Turns out you can just walk out of the map from there.
You get to explore the large city section, its really fun!
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You can go under and above the final boss lair in the Mars strike as well. Once you reach the final building where the ramps go left (to boss) and right (dead end). If you duck and walk towards the wall between left and right ramp you will fall down and can explore. Never did try doing this while the boss was spawned. Might find a hole to shoot him from even though it's an easy strike.