I found an apple airtag in my car.

Hello~ In addition to the excellent information that *Tesserax* has provided read through the link below…

What to do if you get an alert that an AirTag, Find My network accessory, or set of AirPods is with you - Apple Support*

*AirTag, AirPods, and other Find My network accessories include features to guard against unwanted tracking. They should not be used to track people, and should not be used to track property that does not belong to you. Using these products to track people without their consent is a crime in many countries and regions around the world. If an AirTag, set of AirPods, or Find My network accessory is discovered to be unlawfully tracking a person, law enforcement can request any available information from Apple to support their investigation.

Also further down in the link it gives you more tips about what to do…

*Get information about or disable an AirTag, Find My network accessory, or set of AirPods

~Katana-San~

That's up to you. One thing would be to remove the battery so it can no longer function. Whether or not, you want to get law enforcement involved would be your call.

There is nothing anyone here, nor Apple can do to assist you with this. If you do decide to turn this AirTag into law enforcement, they can work with Apple to attempt to uncover the owner.