It's near the the crane controls heading towards Captain Flynt's area when you have to raise Claptrap up.
Basically if you go back a ways from the crane lever off to the right you'll see a small area w/ 2 ammo crates, jump over there then head counter-clockwise making a couple of jumps to new ledges and you'll find the Vault symbol in an enclosed area on the floor near a chest. --- Ilia Iacta Est -Gaius Iulius Caesar
A decent amount of the time, but not always. For instance, there's one in the area where you fight Midgemong that doesn't have anything lootable near it, it's just tucked away on the back of a building. --- Ilia Iacta Est -Gaius Iulius Caesar
Yeah I found one hidden in the prison. In the hidden room with the switches that open the jail cell doors for a hidden fun chest. Pretty sure finding them is tied to a challenge I remember a pop up and I think I "used" it when I walked up to it --- PSN: JManGT GT: James Upton
There's one in Claptrap's Place in the very beginning of the game, and one on a wrecked boat off in the distance in the area where you fight Midgemong. I forget the name of the area, but there's a bunch of Rakks flying over the boat. --- [Exit, pursued by a bear.] GT: LafayetteV