Can Someone Steal My AirTag and Use It Themselves?
Nope. AirTags have a “Pairing Lock” feature that’s very similar to the “Activation Lock” that was added to the iPhone several years ago in iOS 7.
This means that once an AirTag is paired with your Apple ID, it can never be used by another Apple ID unless you specifically release it from your iPhone. Even if somebody opens up the AirTag, pulls out the battery, and resets it, it will refuse to pair with any other device as long as it remains registered to yours.
To release an AirTag — for example, if you can give it away to somebody else — you need to remove it from Find My on your iPhone. As long as an AirTag is listed in your items, it can never be used by anybody else.