Accounting for visitors through the detection of a mobile phone, is it possible?

Question

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Greetings!
We set an interesting task, namely the accounting of visitors.
Everything would be simply solved through IR sensors, or through a camera, BUT you need to determine the duration of the visit ....
WiFi comes to mind, but then you need to have a way to determine available devices without connecting and somehow determine their unique identifier, as far as I know, this is not possible without a network connection.
What other options are there besides pretending to be a mobile tower?

Answer 1

There are no guarantees that a person has a smartphone at all (let's say I have a regular phone), there are no guarantees that Wi-Fi is not hard disabled, there are no guarantees that a person has not simply forgotten his phone at home.
You will not be counting visitors, but spherical vacuum weather on the Moon.
With cameras it is solved - if you add recognition.
https://youtu.be/fanafD_XUno
https://habrahabr.ru/company/croc/blog/151680/
https://habrahabr.ru/company/croc/blog/308300/