Unknown MAC address on my Cox Modem's Wireless Network

I just noticed an unknown device (weird MAC Address of 00.00.ca.01.02.03) connecting to my Cox-issued internet modem's wireless network. I have every device of mine labeled on my network monitoring software, but this one appeared in the last week. I've turned off every device, but it still connects (so something is connecting with the password). Weird IP address as well, ending in .252. I went in today and got a replacement modem (same model, both are Arris). No weird devices....until about 2 AM, when the same weird MAC address suddenly appeared. New modem, new password, everything. Anyone have an idea what I'm looking at?