Now in the cities of San Andreas operatives work in civilian clothes. If you have excellent intuition, you can recognize them. If you commit a crime in a place where there is no police, you can stumble upon agents, and they will act as a police. They are watching you, drive on unusual cars and patrol the streets. In a collision with the police, they also enter into support against you.
Now, upon receipt of stars, additional forces are added to the police. Closing around the city, your intuition will not let you down. So it will be more interesting to play, knowing that there are more eyes in the city.