If it helps anyone, I have noticed that the wifi interface MAC address is the same as the MAC address advertised during the setup phase.
With a tool such as Wi-Fi Explorer you do not only see the SSID farmbot-abcd but you can also see its MAC address (MAC address of the AP): b8:27:eb:xx:xx:xx
The first 3 groups of characters (b8:27:eb) represent the OUI for Raspberry Pi; the rest of the characters are unique for your RPi.