[sratner]

By tracking all devices, noting when one address disappears and a new one appears, and correlating it with the perceived signal strength to reasonably guess whether this is the same device vs. a new one entering your detection radius. On top of that, there is often a fair amount of information besides the address in the contents of the advertisement packet, the set of services this device implements, battery level, manufacturer data fields, ... - much like browser fingerprinting by checking for fonts and canvas edge cases.

Maybe not easy, but also not hard. The only thing that screws you up is someone playing with the airplane mode toggle of their phone while moving within your detection radius.