My understanding is that iPhones can anonymously detect nearby AirTags and help report their approximate locations to Apple, so the owners can find them. When my phone does this, is the information sent over my SIM or cellular data connection? Could detecting other people's AirTags add to my mobile bill, even if the amount is tiny?
2 Answers
The AirTag itself broadcasts an occasional Bluetooth signal. A nearby iPhone picks that up, determines its own approximate location, and securely relays the report to Apple through its internet connection. So it isn’t completely data-free, but the transfer is tiny—probably less than many ordinary actions you perform on your phone.
Yes, the report can use your phone’s internet connection, including mobile data when Wi-Fi isn’t available. However, the amount of data involved is extremely small—so small that you’re very unlikely to notice it on your usage or bill.

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