Your smartphone may have some apps that are continuously listening inaudible, high-frequency ultrasonic sounds from your surroundings and they know where you go, what you like and dislike — all without your knowledge.
Ultrasonic Cross-Device Tracking is a new technology that some marketers and advertising companies are currently using to track users across multiple devices and have access to more information than ever before for ad targeting.
For example, retail stores you visit, a commercial on TV or an advertisement on a web page can emit a unique "ultrasonic audio beacon" that can be picked up by your device’s mobile application containing a receiver.
This information helps advertisers to create your personalized profile and collect your interests by figuring out that both devices probably belongs to you, allowing them to target you with interest-based advertisements.
http://thehackernews.com/2017/05/ultrasonic-tracking-signals-apps.html