The zombies can hear you miles away if you shoot a makarov, that's not realistic enough for you? Anyways, yes, some forests the ZED's does actually spawn in, and yes, it is a good point. But the thing is, when you spawn a zombie, or a group away from a player, there's going to be lag (As this mod is now and the arma 2 configurations are), so everything would be a lagging mess.