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I was under the impression many private sector companies have already deployed these tools.

Just a matter of companies buying/implementing them.

Yes, visit tracking has been around for years. Here is one product (https://support.google.com/google-ads/answer/6100636?hl=en), I assume there are many more.

I guess it's not more well-known because it's not in the stores interests to publicize it.

Any idea how it works? The site is not very helpful in that manner.

Presumably using Bluetooth beacons.

Probably the same way that Google Maps traffic or “this restaurant is busy now” indications work.