Detect a phone nearby without a Wi-Fi connection?

Question

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Good afternoon! Is it possible to somehow detect if there is a phone nearby without connecting it to a Wi-Fi network?

Answer 1

Here again, the whole point is how important the goal of the task is, that is, the amount and complexity of the costs in solving it.
The switched on phone has already been identified and registered in nearby cellular networks (even without a SIM card), and then it lies without emitting anything. But there is also a period of re-registration of the device in them set by these networks, usually it is in a few hours. Or, when the device is moved, the process of transferring a subscriber from one base station to another during a telephone conversation or a data transfer session (handover) is performed.
It is precisely at these moments that the device radiates, and you hear the same sound in the acoustics nearby.
A little more detail about how cellular networks live can be found, for example, in https://habr.com/ru/post/268127/ and https://www.ixbt.com/mobile/gsm-nets.html
So, to detect such a device, you need constant long-term scanning of the environment, or, as they do when searching for spy bugs, they jam the air with interference, turn off the jammer, and then they scan where someone starts to whistle where, re-register)
But about the implementation of the solution to the problem, it’s better to cooperate with Evgeny Ronov, who has already implemented a similar high-frequency radio scanning and described it in the article “Microwave passive radar in the 10 GHz range” - https://habr.com /en/post/248969/