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 monitor changes in Wi-Fi frequencies and identify small patterns of interference, like the kind a person makes when they enter a room.

That means Xfinity Shield can detect people without traditional motion sensors, cameras or video, better preserving privacy while still sending alerts via the Xfinity app if the router notices unexpected human movement. The technology requires not only an Xfinity Gateway device, but also at least one connected Wi-Fi device on the Xfinity platform. I’ve seen it used in standalone sensors, new smart light bulb kits and systems used for aging-in-place arrangements in nursing homes, but never as a free router upgrade.