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. //

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.

As Euclid explains in its privacy policy, "...if you bring your mobile device to your favorite clothing store today that is a Location -- ­and then a popular local restaurant a few days later that is also a Location­ -- we may know that a mobile device was in both locations based on seeing the same MAC Address." //

The defense is to turn Wi-Fi off on your phone when you're not using it.