Chapter 11: U-Boat
Once you reach the first barracks, just after the starting room, go all the way to the end and enter the last room on the right to find this.
Gold Letter Opener:
For this item, just after you exit the starting room, enter the barracks but go to the left after you deal with the enemies to the right. There, beside the charging station will be two doors. One you can pick and one you can hotwire. Go through your door and in the cabin it will be on the desk.
Just after the transition, when you enter this room, go to the left side and you will see the skull under the water against the wall.
Gold Chamber Pot, Gold Toy Robot, and Gold Crown:
When you reach the upper chamber at the end of the chapter, you can find each of these items along the ledge of the circular room.