In Act II in Moresby, there is the side quest "Uninvited Guests", wherein the Hero must kill the zombies in Alicia's apartment. Just before entering the room with Alicia inside, the player should enter the bathroom to the right instead. When they enter the bathroom, to the left is a cabinet with a glass door on it with the Blue Skull inside sitting on a shelf. The player must smash the glass before they can pick it up.
Kill the zombies and free the survivor. He will then lead the Hero inside. In the main room, there is a manhole cover on the ground to the right, which serves as a portal to another bunker on the other side of the map. Once the player goes through it, they will be in a room with a door to the right and a ladder to the left. The player will need to go up the ladder and pick up one of the red propane tanks and drop it down, placing it in front of the door, and the shoot it to blow up the door. Inside the room, there will be a square indentation on the wall with a skull to the right of it, and an empty space for the skull to the left of it.