Comcast is promoting WiFi Motion, part of a home-protection bundle called Xfinity Shield. It detects movement inside your home using nothing but the WiFi you already have — no camera, no motion sensor, no extra hardware.

Before the alarm: it is opt-in and disabled by default, and it has existed since at least August 2024. What changed is the marketing, not the switch.

How it works

You nominate stationary WiFi devices around the house — a smart speaker, a thermostat — and the gateway treats the radio link to each one as a tripwire.

Xfinity's own explanation is the plain one: when a person moves through the space between the gateway and one of those devices, their body changes how the radio signals travel. The system watches for those disturbances and pushes a notification to the app.

This is not a Comcast invention. It is WiFi sensing, a well-established physics result — a human body is mostly water and water absorbs and reflects 2.4 and 5 GHz radio — and it has been in research papers for over a decade. What is new is that it now ships to millions of homes as a product, at no additional cost on compatible plans, with a $15/month tier above it.

What it is genuinely good at

Worth saying, because the privacy conversation tends to skip it.

There is no camera, so there is no footage to leak, subpoena or have a support agent look at. Radio-based sensing sees that something moved, not who or what. For an elderly relative living alone, "no movement detected since yesterday morning" is a useful signal with far less intrusion than a camera in the hallway. For a holiday home, so is "movement at 3am".

Comcast also states it does not monitor the motion or the notifications the feature generates.

The question that is actually interesting

Not whether Comcast is watching. Whether the record exists, and who else can ask for it.

A WiFi Motion deployment produces something that did not previously exist: a timeline of when your home was occupied, held by your internet provider. Not content, not video — a pattern of presence and absence, which is one of the more revealing things about a household. When you leave. When you come back. Which nights nobody is home.

Comcast's documentation reserves the right to disclose information to third parties without further notice in connection with law-enforcement requests or court orders. That is standard telecoms language and it is not sinister on its own. It becomes interesting because it now applies to a new category of record. And the documentation does not say what is retained or for how long, which is the one answer that would settle the question.

The pattern is familiar from Apple's fight over a UK technical capability notice: the argument is never about the feature, it is about what compelled access to the resulting data looks like later.

The practical limits

Three, all acknowledged or obvious:

  • It cannot tell a child from a large dog. Sensitivity is a signal threshold, not a classifier.
  • Radio does not respect walls. In a flat, the neighbour's movement is a plausible source of a notification.
  • It only covers the paths between the gateway and the devices you nominate. Coverage is a set of lines, not a volume.

What to do

  • Know that it is off unless you turn it on. If you did not enable it, you do not have it.
  • If you want it, ask the retention question first. Support can be asked how long motion events are kept and whether they are stored on Comcast's systems or only on the device. Get that in writing before enabling.
  • Think about who lives there. Housemates, tenants and guests are inside the coverage of a feature one person enabled. A record of when someone is home is not only about the account holder.
  • If you rent, remember the router is often not yours. A landlord-supplied or building-supplied gateway is a different consent conversation entirely.
  • For elderly-care use, weigh it against the alternative honestly. Compared with a camera, this is the less intrusive option, and that comparison is the fair one.

What is not established

  • What is retained, and for how long. Comcast's documentation does not say.
  • Whether events are processed locally or in the cloud. Not specified.
  • Whether any law-enforcement request has been made for this data. None reported.
  • How accurate it is. No independent testing of false positive or negative rates has been published.