● Minimap with important marks:

● Start the mission, do the first few objectives, and before you go for agent Salter, go to the hidden room
 with the golden bars. It's door is in a large painting of a forest on a wall, near the large room with
 the officers and armors.

● Get two bars, 12 kg.-s each, so you have to drop a few things. As i know the order
 is important too, so better get a carachter who can carry two bars.

● Go to the library to agent Salter with the bars. Drop one bar in the library, and get another one from the
 secret room.

● Now back to Salter, talk to her and head to the Archives.
 As you follow her you will see a soldier cleaning a window, on the right side there is another hidden door
 in a painting
● It's the room with the stolen paintings, drop a bar here too.

● Then go to the Archives and drop the last bar here.

● The reward is on the yard by the truck.