Knife

General Information
The knife is a weapon that can be picked up by the player. Clicking while equipped will result in animation where the character stabs itself in the head. then you die. it is most likelly a reference to Spy knife from TF2

Location
The knife can be located right next to the AWP inside the Sniper Shack on top of the mountain. It is stuck inside the wall, which can indicate that it was used before. it can be picked up by simply clicking on it.