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.

Item	Location
Hidden box - Abandoned Factory	To the west (left) behind the barricade on top of some pipes.
Hidden box - Bridge	North- Infront of you and to the upper right you will see it on top of the bridge wall.
Hidden box - Forrest	North- The Clip on flashlight is in front of you on the front tire of the large truck.
Hidden box - Forrest	South- Behind you in the distance is a car with the body armor crate in front of it.
Hidden box - Forrest	South/West- Directly behind you is a fallen tree with the explosive ammo crate peaking out to the right of it.
Hidden box - Storm Drain	North- Right in front of you next to the flaming barrel is the Armor piercing ammo box.
Hidden box - Storm Drain	South- Directly behind you there is a far off flaming barrel with the mooghido pistol box next to it.
Hidden box - 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.
Hidden box - Lab Basement	Hallway to the right of the long hallway (east) with the stairs in the distance.Above where the ceiling pipes run.