Heres an example map for how to use the Ai_Goal_Police entity in HL2. The entity basicly does is it has a radius around itself. You pick a metropolice entity to be affected by this entity. At the start of the map, or when triggered, the metropolice will walk to the center of the entity. Then, when the player walks into the radius, the metropolice will stop the player and attack the player if needed.