Yeah the grass flattening works, but thats the only real thing you got... and its not like you can track somebody for 15-20 minutes on the off chance that they prone their way across the map. Just to have things that are hard to spot unless your looking for it... Tree branches bent if a player runs too close, footsteps in mud, have them drop an empty can if they eat or drink from a can, shell casings from guns, etc... It would be bloody hard to do, but the result would be amazing in my books...