C6300 - Unknown MAC address keeps appearing to login in logs

Hello,

After having some issues with my router dropping its signal at seemingly arbitrary points, I decided to look in the router's logs for an answer. While that issue seems to be with some bizzare ICMP floods, I also noticed that a strange, unknown device was connecting and reconnecting to my network in the middle of the night. I looked up its MAC address, but none of the lookup sites could tell what vendor this device had. I've changed my WiFi password since, and even enabled Access Control so that no new devices could connect. Any ideas?

Thank you in advance.