Wi-Fi capable devices, like smart phones, often automatically send out probe requests in order to detect available Wi-Fi networks in the area. These probe requests include a device identifier known as a MAC address. Nowadays, most phones, as well as some other devices, utilize randomized MAC addresses to increase privacy, as a non-randomized

Source: Hot Hardware – Hackers Can Track You And Steal Your Data With Wi-Fi Probing, How To Protect Yourself