S.T.A.L.K.E.R. had an alright spawning method. Basically, if there's a void, it will be filled. As areas were cleared, new NPCs were respawned outside of the accessible map. They walked in to fill the void areas. The longer you left an area unattended, the more dangerous it would become as more NPCs moved in. Some NPCss would set up camp and stay put. Others would walk around the map and help clear areas they passed through.
