My current best guess is, basically what you've suggested. The Amazon Go page says "Computer Vision" which means "Cameras, Cameras Everywhere!". It also says "Sensor Fusion," I bet that means at the very least "Wifi and Bluetooth," with which they'll use to place your location on a virtual map of the store. That location could then be compared with the computer vision object tracking location to "double check" that you really are at that location. And so on.