The reps might be able to give you more of an idea what the particular device is, though they may take a day or two to reply.
I'd turn each device off one by one, or at least the WiFi of the device. Then check to see which device disappears from the list of connected devices. You should eventually be able to determine which one it is.
If it's something eating your data without you actively using it, check things like cell phones, Fire TV sticks, Rokus or even a satellite TV receiver. Those devices can do updates on their own, though with the phones it depends on your settings.
