That's an interesting one.
Finger in the air type of guess:
- The user connected to Wifi (I've sometimes seen the dashboard confuse a wireless user for a wired user)
- The device is using a randomized MAC, so likely an iphone or Android phone - although any device really can do this if setup to do so.
- It's possible the device either has a static IP address set, or that's the IP address it had last used
- Lastly, no idea why you wouldn't see any logs for the association of the client.