Can I use an AirTag that’s been found?
I found an AirTag in my car can I use it?
iPhone SE, iOS 15
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Basically no. It would be associated with the owner's Apple ID, and unless they disassociate it, it will be useless to you.
Instead, you can check to see if that owner provided any contact information so you can let them know you found it.
Ref: What to do if you find an AirTag - Apple Support