There are 3 gnomes hidden in the map that once found allows you to buy a trap that works once a round and kills all zombies for 30 seconds within a certain distance of the middle gnome. Shooting all 3 gnomes on round 1 will give you enough points to use the mystery box.

Gnome 1 can be found outside the wall, to the left of the road laying in some boulders.
https://imgur.com/WneufVU
https://imgur.com/JLxXHfQ

Gnome 2 can be found inside the wall, in the arched rock formation out of the playable area.
https://imgur.com/bxSDoWP
https://imgur.com/vbfg1en

Gnome 3 can be found inside the wall, under the scaffolding to the left of the road next to the wall.
https://imgur.com/qOAmXEU
https://imgur.com/WayBinI