At the western side of OGC you'll come upon a small shack with a diamond briefcase inside it. To get inside the shack, jump onto the large rock sitting nearby and jump onto the roof; once there you can use the ladder to gain entrance and loot the briefcase.
The briefcase here is located on the roof of the building furthest to the west at "Private Property". To get to it, jump onto the crates that are located between the two small buildings, and from there onto the roof. Move across the ladder to loot the briefcase.