@Mikey_Noble -- what is the MAC-address (not the IP-address) of the "rogue" device?
There are websites where you can enter the first six characters of the MAC-address, to see which company has been assigned the MAC address. From that company, you might be able to deduce which device has connected.
Do you have a MoCA filter connected between your cable-modem and the wall-socket, to prevent MoCA signals from computers in nearby homes from connecting to your cable-modem?
If you disconnect the coaxial-cable from the cable-modem, does it change the status of the "rogue" device to "offline", and thus eligible to be deleted?
