There are nine Weapon boxes, you unlock them on your Map if you unlock all ctOS Towers. After you looted the 9th Tobias Frewer will call you (only if you progress in the Story trough Mission One Foot in the Grave (Mission 4 of Act 2) and you can play the Weapons Trade Investigation Mission.

Unlocks: After found the 7/8 Weapons Stash you unlock the Spec Obs Goblin (4 Star, Burst-Fire Silenced Assault Rifle, 30 Bullets)