Thanks to Xandem, this is now possible.

The firm makes sensors that are deployed in networks and utilize radio waves to detect anything that moves within the area covered by the network. Since radio waves are a form of electromagnetic energy that can penetrate solid materials, the sensors themselves can be hidden in walls.

About one year ago, Xandem started selling to beta customers, and this summer it will offer mass market versions of its products, which are currently targeted at the security market.

But such sensors could also be used for home automation purposes, or to support innovative customer experiences. I can imagine a system that warns you that your child is not sleeping well (four times last night, she got out of bed) or reveals your husband is opening the refrigerator in the middle of the night.

With Xandem’s approach, the individual sensors don’t know anything. None of them alone can trigger an alert. Here’s how the firm explains it:

Each of the devices communicates with the others, forming a dense sensing web that blankets the area. When people enter the network area, they disturb the radio field and Xandem’s powerful algorithms detect the disturbance. We call it “Synergistic Sensing” because it’s not the individual transceivers doing the sensing, the network itself is the sensor working in tandem.

Think about what will happen when systems like this one are deployed in retail stores, office buildings and public places. You won’t be able to tell whether the location has security, or not.