I need help finding an unwanted AirTag on my car

According to Stay safe with AirTag and other Find My accessories:

Both AirTag and the Find My network are designed with privacy at their core. AirTag and Find Mynetwork accessories have unique Bluetooth identifiers that change frequently. To discourage unwanted tracking, Find My notifies you if an unknown AirTag or other Find My accessory is seen moving with you over time by sending you the message “[AirTag] or [Item] Detected Near You.” (This feature is available on iPhone, iPad, or iPod touch running iOS 14.5 or iPadOS 14.5 or later).

If you hear an Air Tag make a sound:

When moved, any AirTag separated for a period of time from the person who registered it makes a sound to alert those nearby. If you find an AirTag after hearing it make a sound, you can use any device that has Near Field Communication (NFC) technology, such as an iPhone or Android phone, to see if its owner marked it as lost and help return it. If you feel your safety is at risk, you can contact your local law enforcement, who can work with Apple. You might need to provide the AirTag or its serial number.