So palmer lucky is developing a headset for military use using friendly fire tracking i believe it is called eagle eye, you could implement something similar for the soldiers heads up display, if a teammate walks behind cover a rectangle would appear to indicate where the frendly is walking, enemy would also show up if they have been spotted by a team member. if you take a look at some of the videos you will see this could be a way to show indicators that would actually fit the game lore.