Wi-Fi Hotspot Tracking
Free Wi-Fi hotspots can track your location, even if you don’t connect to them. This is because your phone or computer broadcasts a unique MAC address.
What distinguishes location-based marketing hotspot providers like Zenreach and Euclid is that the personal information you enter in the captive portal — like your email address, phone number, or social media profile — can be linked to your laptop or smartphone’s Media Access Control (MAC) address. That’s the unique alphanumeric ID that devices broadcast when Wi-Fi is switched on.
MAC addresses alone don’t contain identifying information besides the make of a device, such as whether a smartphone is an iPhone or a Samsung Galaxy. But as long as a device’s MAC address is linked to someone’s profile, and the device’s Wi-Fi is turned on, the movements of its owner can be followed by any hotspot from the same provider.
The defense is to turn Wi-Fi off on your phone when you’re not using it.
EDITED TO ADD: Note that the article is from 2018. Not that I think anything is different today….