I rather like the footprint tracking idea. Even as a non-pker it seems to make sense that you could tell if someone had been near by recently. Not sure about the first idea though. I think I get it, but you refer to points A,B, and C on the picture but the picture isn't labeled. I'm not sure all of these should be tied in with ranging, but perhaps some sort of tracking skill? Needs some tweaking, but a solid idea I think.