Can you please update your community profile to include your:
We'll then look into this further. Please do not post this information on this thread. Once you've updated your profile please post in your topic to confirm it's updated.
If you are looking at "Active" devices then they should be active at the time you look, that is certainly the case when I check devices connected to my router.
One way to find out what the unrecognised device is, is to force it to disconnect, eg blacklist it, and see what fails or who complains. Alternatively if you can find its MAC address, displayed by some routers, then you can find out who the manufacturer is by entering it into a tool such as this:
this may then give you an idea as to what the device is.