Start The Golden Lion level and go near the garage called "Wagen Fuhrer". Near that garage is a sign with a small arrow underneath reading "Van Uchelen Meubels". Northeast of that sign is a green double door window. Shoot in between the bottom corner of the green window and the top right corner of the sign. There is invisible glass floating in midair at that position, and when shot, will leave bullet marks. Note: This trick only works at certain angles, which there are many. You have to be to the right of the sign and the window for it to work; or you can stand far away and use the sniper rifle to hit it as long as you are on the right side.