The Federal Trade Commission has sent warning letters to 12 Android app developers apparently using controversial "SilverPush" ad software. When used, the software can detect so-called "audio beacons" inaudibly emitted by devices like televisions, giving third-party advertisers more information on viewing habits. The FTC does not name the app developers, but says the developers were sent warning letters after it was determined that their apps on the Google Play Store were apparently using SilverPush code. Although, as the FTC notes, the creators of SilverPush say the software is not yet used in the United States, the warning letters say the apps are requesting microphone permissions without a clear need for them, and do not appear to properly notify users. The developers may be in violation of the law if users aren't notified about what information the apps are collecting, the FTC notes.