Smartphones with WiFi can now be used as an indicator of customer presence thanks to a WiFi mechanism that is common across all such devices: probe requests. These 802.11 management frames are transmitted at regular intervals from WiFi devices. The frames contain information that can be used to identify presence, time spent, and repeat visits within range of a WiFi access point. These devices can be detected by WiFi access points irrespective of its WiFi association state meaning that even if a user does not connect his or her device to the wireless network, the device’s presence can still be detected while the device is within range of the network and the device’s WiFi antenna is turned on.

I’m just a developer, so admittedly I’m unsure if the frames they refer to have any uniquely identifying information. Hopefully not. At the very least they get a heat map of where phones are in the store.

If you connect to the wifi they will get your MAC address for sure, but that’s usually randomized every time you connect.

Hypothetically if you hop on their wifi they can track your device for the duration of that connection all the way to the register. Then you use your credit card or rewards programs and they can put it together. (When I swipe my card at microcenter they always say “do you still live on…” since I’ve purchased online from them.)

Locally at one of our sports stadiums you swipe your card walk in to a snack area, grab what you want, and walk out with it. They use cameras and other sensor to make that possible. If they can figure it out Walmart certainly can.

This is all tinfoil hat stuff based on pieces of information I have, but it sure is interesting to think about.