When I look at my devices in the BT app, most are easily identified. However there is one which is decribed as "unknown". Its Mac address is 9A:59:F6:A4:A8:54. When I look it up in a Mac address finder, it comes back as "Not Found". However, whatever it is seems to have been pretty busy as it seem to have downloaded about as much as everything else added together. I should just say that it is connected to my Complete wifi disc, though I am not sure that that makes any difference.
If it has downloaded about as much as everything else put together it may not be a device such as a phone/tablet or laptop.
If you do a lot of streaming it could be a Smart TV
If not a TV are you using Powerline Adaptors?