You will most likely get this without noticing.
I got this on Freeform Training, I subdued a guard which made it so his pistol dropped on the floor.
When you get rid of the subdued guard (by dumping him somewhere hidden or putting him in a closet/chest), arrange matters so another guard walks near by and sees his lone pistol on the floor.
A question mark will show above their head and they will move the weapon automatically into a hidden stash and the trophy will pop!