Hundreds of Android apps could be covertly tracking users via inaudible sounds emitted by nearby devices, researchers have found.

Researchers discovered technology that lets devices talk to one another for tracking purposes using ultrasonic tones on 234 Android apps.

Televisions, billboards, websites and shops can emit the high frequency sounds, which can't be heard by humans but are picked up by the apps. This signals whether a person has engaged with an advert by watching it, or visited a shop, and how long for.

Apps featuring the technology include those from McDonald's and Krispy Kreme. Major companies could be using it to track customers' location and habits, both on and off their mobile devices, without them knowing, the researchers warned.

"An adversary can monitor a user's local TV viewing habits, track their visited locations and deduce their other devices," said the researchers. "They can gain a detailed, comprehensive user profile with a regular mobile application and the device's microphone."

The tracking method has spiked in popularity recently, according to the researchers. Two years ago just five apps in the Google Play store used the technology. Now, it is allegedly present in 234.