Three probe requests

When their wireless is switched on but not already connected to a network, mobile phones emits a signal every few seconds to scan for known wireless networks in the surrondings. To do, the phone shouts its unique identifying address. In the vicinity of a footfall system, that shoutout is heard, the unique address is registered along with a timestamp, location, signal strength, device manufacturer and in some cases a list of other networks that the device has connected to recently. The collection of this data falls under the European GDPR, as the unique mac address can be considered as private data.

To see your MAC address:
Android: open Parameters, select about this Phone. The wireless MAC address is listed under Wi-Fi Address.
iPhone: open Settings, select General, then About. The wireless MAC address is listed under Wi-Fi Address.

More recent phone operating systems employ obfuscation techniques to reduce the chance of being tracked and identified, hiding the unique identifier and the names of the known networks in the probe requests. Nonetheless, they have been used by police to search for suspects.