Thanks for all the replies. I turned off the router, turned Bridge Mode OFF on our ISP modem/router so we can go back to that for a while. So far, no strange MAC addresses. It sucks cause they don't...

My wife dicovered we've been using 3x the amount of Data our ISP allows us. That is VERY unusual. Looking at the router info, there are two "locally assigned" MAC address that have unknown IP...