JDMurray said: Is your router a switch too? A device "connects" to a switch at Layer 2 by sending just one frame that puts the source MAC into the CAM table. Is it possible that someone tried to cable-connect a MacBook to your router to get Internet access? Does Apple publish a list of its OUI blocks that identify which Apple device would/could be assigned that MAC address?
JDMurray said: Some devices have the capability of using a software-configurable MAC address (aliasing) to be used in place of the burned-in MAC address (BIA) that's encoded in the device's NIC hardware. Network routers and Wi-Fi Access Points can be configured to use a source MAC address alias. Might you have plugged anything in to a switchport that supports MAC aliasing?