To complete this, you need to do the ambient freeplay in Saabrucken. This is the area in the top right of the map. Access to the area is via a gate where you need to push a button to open with 3 guards standing in front of it.
Like other descriptions for trophies of this category, the description is wrong. You need to complete 55, not 53, of the 58 freeplay events.