Researchers are warning that ads could play coded sounds outside the range of human hearing to secretly communicate with other gadgets within earshot.

The technique, which several companies are reportedly working on, would allow marketers to associate devices with one another and paint a privacy-cracking picture of the owner's interests and behaviors.

Dan Goodin reports that cross-device tracking is already in use:

Cross-device tracking raises important privacy concerns, the Center for Democracy and Technology wrote in recently filed comments to the Federal Trade Commission. The FTC has scheduled a workshop on Monday to discuss the technology. Often, people use as many as five connected devices throughout a given day—a phone, computer, tablet, wearable health device, and an RFID-enabled access fob. Until now, there hasn't been an easy way to track activity on one and tie it to another. "As a person goes about her business, her activity on each device generates different data streams about her preferences and behavior that are siloed in these devices and services that mediate them," CDT officials wrote. "Cross-device tracking allows marketers to combine these streams by linking them to the same individual, enhancing the granularity of what they know about that person."

The trick hasn't been seen in the wild, but all the pieces are in place: we all know our smartphones and laptops might end up under someone else's control, but did you know television sets now default to collecting and sending data on what you watch?

[via The New Aesthetic]