Comcast on Monday announced Xfinity Shield, a home-security platform that taps into Xfinity Wi-Fi in new ways. It adds WiFi Motion, which uses compatible Xfinity Wi-Fi routers to detect movement in the home without cameras or dedicated motion sensors. WiFi Motion and other Shield features are available at no additional cost to eligible Xfinity customers, although compatible equipment and other requirements apply. I know it sounds eerie to think about a Wi-Fi router that can see you, but I’ve covered this presence-sensing technology before, and the goal is the opposite of invasive surveillance. This kind of technology uses algorithms to…