The police went to catch the bad guys. Why did they whistle? Didn't the bad guys hear that and run away? It would be nice to go quietly, without anyone knowing.

Actually, it's not like this, because they all catch people purposefully. When approaching, they will turn off the siren, not always on. The road is in poor condition, so they honk their horns to catch up with time and prevent the bad guys from escaping. When the road is clear, you can catch people and drive very fast, but you don't have to whistle. The siren is handled by the case handlers according to the needs.

Or the bad guys have been arrested before going, and the police car only goes to escort, so it doesn't matter.