I'm having this same issue. I see a computer showing up named "Full_Ford", and it lasts for about 30 seconds. I've taken the MAC address and cannot find it in my router's list of known devices. Not knowing what this was, I changed my wifi passkey to something that is VERY strong, 12 characters, with a various mix of special keys and numbers, blah blah blah. I also disabled my guest network.
This morning I got up and saw the same thing again. the description shows it as an Amazon item, model #KFFOWI. Google search shows it as being a tablet, although I have no Amazon tablets.