There are hidden boxes hidden around the levels that will unlock new guns, ammo, etc. North will always be where the menu shows up. Also the boxes will not show up until the round has started.
Abandoned Factory – To the west (left) behind the barricade on top of some pipes.
Bridge – North- Infront of you and to the upper right you will see it on top of the bridge wall.
Forrest – North- The Clip on flashlight is in front of you on the front tire of the large truck.
Forrest – South- Behind you in the distance is a car with the body armor crate in front of it.
Forrest – South/West- Directly behind you is a fallen tree with the explosive ammo crate peaking out to the right of it.
Storm Drain – North- Right in front of you next to the flaming barrel is the Armor piercing ammo box.
Storm Drain – South- Directly behind you there is a far off flaming barrel with the mooghido pistol box next to it.
Storm Drain – North/West- The first hallway to the left of the flaming barrel (where you found Armor piercing ammo) there is another box far down the hall.
Lab Basement – Hallway to the right of the long hallway (east) with the stairs in the distance. Above where the ceiling pipes run.