In the Southern Shelf area there is a mini "fort" for lack of a better term next to the small bandit town (near the first Handsome Jack audio recording). Inside the bandit town there is a drawbridge that is up and a sign that says "scram". Drum music can be heard around this area. It appears to be the only way to access the adjacent "fort", but I can't figure out how to make the bridge drop so I can cross. I also can't find any reference to this area anywhere on the internet.
Access to this fort will open automatically as you progress through the main quest. Claptrap will lower the drawbridge eventually :)