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.
Happens all the time.
Both iOS and Android devices use MAC Randomisation security which is why you can't look it up, it's a made-up address
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?
The OP quotes a MAC starting 9A, the A suggests that it is an Apple device