You can display the 16 bit addresses in the device list page by clicking the grey eye in upper right corner. Look for that address, which may or may not be found.
You could try shutting down the hub, pulling power from the wall(not the hub) and then restart the hub-which may clear the neighbor table
@Rxich thank you for the tip on using the "grey eye" to display the 16 bit adresses! Unfortunately after shutting down and removing the power from the wall the Unknown decive was still listed in the log for one repeator.