Attached Devices in basic setup

> How can I determine what the device is as the reference to it is
> unknown?

   "reference"?  Thanks to your detailed report, it's certainly unknown
to me.  Wired or wireless?  Radio band?

   These days, a MAC address typically identifies a chip, which is
typically sold to multiple gizmo makers, so it might be unrealistic to
expect to identify a whole device based on a MAC address.  But there
exist web sites where you can take yours (or the top half of it,
anyway), and see what you can learn.  For example:

      https://dnschecker.org/mac-lookup.php

   If I were trying to identify some mysterious device which appeared in
an Attached Devices report, then I might try some basic detective work,
like, say, pointing a web browser at its IP address, to see if it
responds with some useful information.

   Failing there, I might try powering on/off every "smart"
Internet-of-Junk device which I could find (or, if desperate, section of
the house), to see what comes and goes.