At the start of the game, you will find three purple markers on your map. These are the locations of your former squad. When you find these locations, you can then take the trackers from the bodies. Now you have them, you can freely place them in items and people on the map. This allows you to see their position or movements when you open your map.

If you get one and place it on Virginia while she is allied to you, you can then track her movements when she runs off to scavenge. You will also earn this achievement for doing so.