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>tracked using ambient Bluetooth signals from phones.

What does this mean? Are they just measuring radio power levels in the 2.4GHz spectrum, and using that to determine whether a car is there or not? Or are they actually somehow capturing bluetooth signals, decoding them, and extracting identifiers (eg. MAC) from them?

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It was explained to me as if they track individual cars so I assumed they extract identifiers. I don't know much about BT so I had no reason to question if that was possible. I asked about TPMS as an alternative and they said bluetooth was more reliable since many cars didn't use in-wheel transponders.

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