It’s a bell on a string.

That’s an easy way to describe Comcast’s Wi-Fi Motion, a recently announced home technology feature that lets users detect movement within their residences.

It starts with their modems or “gateways,” which are sending out continuous Wi-Fi signals. An intruder getting in the way of the Wi-Fi waves causes a disruption. This triggers a message to the homeowner — a ringing bell, if you will.

Wi-Fi Motion isn’t new. Philadelphia-based Comcast made it available in 2024, but says it chose not to publicize it as it worked to perfect it.

Last month, Comcast re-released Wi-Fi Motion as part of a multi-pronged security suite, called Xfinity Shield. Included in the package are cybersecurity tools and parental controls.

Wi-Fi Motion is getting the most attention, though, partly because it’s seen as a genuine breakthrough.

“Wi-Fi Motion is probably the biggest innovation in this space in the last several years,” said Kohposh Kuda, Xfinity senior vice president of consumer products.

Also, Wi-Fi Motion has raised privacy concerns — partly because Comcast doesn’t pledge to keep Wi-Fi activity private in all instances.

“Comcast may be required to disclose information generated by Wi-Fi Motion in response to a valid subpoena, court order, search warrant or other lawful legal process, as required by law,” an Xfinity-app help page reads. “In limited situations permitted by law, information may be disclosed in connection with legal proceedings, dispute resolution or emergency situations.”

Comcast adds that it “does not monitor the motion and/or notifications generated by the service.” It doesn’t say whether it’s technically capable of doing so.

Wi-Fi Motion stores seven days of activity in Xfinity’s cloud system when authorized by the user.

Wi-Fi Motion doesn’t use cameras, capture images or video, or record audio. It doesn’t identify individuals.

“Although Comcast says that Wi-Fi Motion captures only motion activity, and does not identify individuals, that’s a choice, not an inherent technology limitation,” writes Adam Engst, editor of the TidBITS technology newsletter and website.

“Earlier this year, German researchers demonstrated that Wi-Fi signals could identify specific individuals with 99.5% accuracy,” Engst notes.

Comcast takes pains to emphasize that Wi-Fi Motion is opt-in — It can be turned off with a tap or two in the Xfinity app at any time.

Those unconvinced that they are secure can take the extra step of putting their gateways in “bridge mode” and plugging in third-party modems, thereby bypassing Xfinity Shield entirely.

Xfinity Motion is free but only works with certain recent-model Xfinity modems.

It requires other fixed-in-place Wi-Fi devices in a home to work properly. A reporter setting up Wi-Fi Motion in his home office availed himself of an Amazon Echo home speaker and a Wi-Fi-connected Canon printer to create a Wi-Fi Motion zone — three bells on strings forming a tinkling triangle.