Chapter 8: Camp Belica
Accept the side mission to kill the commander from the guy hanging out in the courtyard under tree and when you return to him to let him know the commander is dead, he will give this to you.
Accept the side mission to find the medicine from the couple in the bunks near the start of this mission and when you return to them with it, you will receive this.
Once you have returned to Set and made your way to the Camp Command building, climb up the wooden ramps to the top and enter the office on the right. You will need to get a safe combination from the desk drawer and then pop the painting of the wall at the other end to find the safe. Open it and retrieve the familiar gold gun.
Go down another floor from the previous code and as soon as you reach the landing, look for the bust on table to the right, against the wall.
At the very end of the chapter when you are to jump in the truck with Set, go to the left instead and enter the office there. Look in the desk drawer to find the mask there.