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What transmission is still active when the phone is off?

Both the UWB tracker and the NFC radio can be on even when the device is "off" (although there's settings to change that behavior).

IIRC the baseband can potentially do all kinds of crazy stuff too (no evidence that iphones actually do anything unusual, but perhaps that could change if the government sent a very compelling court order)

can't say exactly, but the "find my" feature explicitly remains active even while "powered off" so it's evidently enough transmission that Apple can track location.

Apparently this uses the phone NFC capability and other nearby active phones to determine location. So yes, either leave the phone at home if you don't want to be tracked, or use a faraday bag.

That is option that you need to turned on.

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