The makers of 12 Android apps were sent a warning letter by the US Federal Trade Commission (FTC) because they included code which could enable third parties to monitor users’ TV viewing.

The software, called Silverpush, uses the device microphone to listen for audio signals (or beacons) in TV ads in order to produce a detailed log of the TV content viewed. This can then be used for targeted advertising and analytics.

Although Silverpush said its service is not currently in use in the US, it encouraged developers to notify when apps could allow third parties to monitor viewing habits should it begin operations in the country.

The FTC notes that while the unnamed apps (available via Google’s Play store) ask users for permission to use their device’s microphone, despite not appearing to need it to function, they do not mention that they could monitor TV-viewing habits, even when the app is not in use.

The letter warns developers that if they imply that their apps are not collecting and transmitting TV viewing data when in fact they do, they could be in violation of Section 5 of the FTC Act, which “prohibits unfair or deceptive acts or practices in or affecting commerce”.

“Upon downloading and installing your mobile application that embeds Silverpush, we received no disclosures about the included audio beacon functionality,” Maneesha Mithal, associate director, division of privacy and identity protection, wrote in the letter.

“We would encourage you to disclose this fact to potential customers, empowering them to make an informed decision about what information to disclose in exchange for using your application,” she wrote, adding that FTC staff will continue to monitor these apps in the coming months.

This is not the first time the FTC has taken action against app makers. In April last year, it fined Health Discovery Corporation, maker of MelApp, which claimed it could help diagnose or assess melanoma risks in users.