You will find hidden caches of equipment through the levels "Sundown" and "Nightfall." You will need to find all caches within these two levels for the achievement to unlock. Most of these caches will be marked by a nearby Ranger logo. There is more than just the Ranger caches in this level. Below are the locations as follows. They do not have to be collected in any order, however, be sure to collect everything before leaving Sundown.
Just below the guard tower is a turned up car.
Inside the guard tower inside the compound.
On the counter, next to the gas canisters within the Gas station.
Within a small room past the gas station. There is a small hole in the wall you will need to enter.
On the roof of the Gas station is a small hole where you'll find a cache.
Within the cockpit of the plane.
Move onto the plane and across the wing.
Next to the generator for the ferry.
Inside the back of a white truck within the compound area.
Second floor of the house in front of you, as you leave the ferry.
After the Giant Shrimp charges you, check the back of the truck in front of you.
Inside the Mall bathroom after entering the second floor.
Across the room from the bathroom, will be a room with three traps. Within this room.
Before jumping out of the Mall, look down off to the left to see a truck. Within this truck.