When your teammate sees an enemy, that enemy's location becomes available to all the teammates on the radar.
Yea which is also why i don't accuse people of wall hacks, simply because i can't see the enemies radar

I have been playing with him the past like 6 days and I haven't seen him wall at all. And I spec him all the time because it's fun to watch. Every person that he has known is somewhere, I knew they were there also from his perspective.