Comcast is bringing Wi-Fi motion sensing to millions of routers that are already in customers’ homes, turning the devices into activity monitors. A new update to the Xfinity Internet app, arriving today, August 18th, enables the feature on compatible Xfinity routers at no extra cost.

Announced as part of its new Xfinity Shield service, WiFi Motion can monitor activity in your home and send you notifications, similar to a standalone motion sensor. It works by detecting disruptions in the Wi-Fi signal between the Xfinity gateway and any Wi-Fi-connected device, giving you some insight into what’s happening in your home without buying additional hardware. Xfinity Shield is free for all customers and includes a new advanced security feature that protects devices from online threats, including malware and phishing attacks, and improved parental controls.

Comcast’s motion sensing pairs with new modes in the Xfinity app — Home Watch, Away Watch, and Dark Watch (a night/sleep mode) — allowing you to turn sensing on or off based on your activities. WiFi Motion requires an advanced gateway, XB7 or newer. It’s opt-in and can be turned on and off through the app.

Kohposh Kuda, SVP of consumer products at Xfinity, told The Verge that the company has put in a lot of work fine-tuning Wi-Fi motion sensing. “We have spent the last three years advancing the algorithms for WiFi Motion and tightening up some of the irregularities that came with the early versions of Wi-Fi sensing. We’re at a point where we feel confident in the WiFi Motion capability that customers can use this for motion detection in their home and for a basic level of security.”

Xfinity doesn’t see the technology as replacing traditional security systems with cameras and physical motion sensors just yet. “But for the average household … this provides an adequate level of security protection,” says Kuda. Xfinity already offers security cameras, and those remain available under a new service called Xfinity Shield Select. This costs $15 a month for an indoor camera and a door/window sensor and includes cloud video storage capabilities and a 24/7 urgent response service.

The potential uses for a reliable motion-sensing technology that doesn’t require additional hardware are broad, from home security to aging in place. Several companies are starting to develop solutions in this space, including ADT, which recently acquired Origin Wireless, one of the original Wi-Fi motion sensing companies. Comcast also has bigger plans for the technology. “We believe this is the foundation for what we think we can do with more advanced use cases,” says Kuda.