Found an IPhone
What do I do if I found someone’s iPhone but can’t get any information, to return it to its owner?
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If the device is able to connect to the Internet, you can try holding down the home button (or right side button on models without a home button) to talk to Siri, and say "whose iPhone is this?"
If it is not online or if that does not work, I'd recommend turning it into local authorities.