Unknown Bluetooth Device

2019 iMac, macOS 11.4

An odd one this - a Bluetooth device keeps appearing in my list of available devices. Only its mac address shows 58-e2-8f-5f-d3-74.

If I click on it to connect, nothing happens. If I control-click and select remove, it vanishes but, after a short time, perhaps 10 seconds, it reappears with the same mac address.

I've no idea what this might be. I've not knowingly brought any new devices into the house recently which might have a Bluetooth capability.

I see this as a potential security risk and would like to track it down and eliminate it - one way or another.

Any suggestions as to how I might find it?