Back 4 Blood’s secrets don’t really give you much unless you are a completionist and/or like to hunt achievements or trophies. These golden skull trophies don’t change the game at all. All you have to do is interact with them, and that is it. The third secret in the game is located in Act 1, Special Delivery within The Dark Before the Dawn. Here is where to find it and earn the Pallet Cleanser achievement/trophy.
When you enter the Special Delivery mission, you will first go down a path similar to the one you did in A Clean Sweep. Go past the gas station, and down the street will be a building you can cut through to reach an alley. To complete this mission, you are supposed to find two supply boxes and deliver them to the safe room. While you do that, find this dumpster in the alley by a fence to the area where the safe room is. The area is very small, so you shouldn’t have problems finding it.
Hop on the dumpster and climb on to the storage container. In front of you are two pallets and the golden skull trophy you need to interact with is on the furthest one. Simply jump over to it. If you fall, just go out and climb back up.