The Ammo Box (aka Ammunition Box) is a storage device containing various ammo types and can be found in many of the missions for The Operative: No One Lives Forever, No One Lives Forever 2: A Spy in H.A.R.M.'s Way and Contract J.A.C.K.
In NOLF1, the box is white with UNITY written on the top of the box.
In NOLF2 and Contract J.A.C.K., the box looks more like a traditional military ammunition box. Green with a locking case.
When an ammo box is empty, the box will disappear. If there are still items in the box, the box will remain.
Ammunition box on Wikipedia