uXDT is the practice of advertisers hiding ultrasounds in their ads. When the ad plays on a TV or radio, or some ad code runs on a mobile or computer, it emits ultrasounds that are picked up by the microphone of nearby laptops, desktops, tablets or smartphones.

SDKs embedded in apps installed on those devices relay the beacon back to the online advertiser, who then knows that the user of TV “x” is also the owner of smartphone “Y” and links their two previous advertising profiles together, creating a broader picture of the user’s interests, device portfolio, home, and even family members.
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Their results revealed Shopkick ultrasonic beacons at 4 of 35 stores in two European cities. The situation isn’t that worrisome, as users have to open an app with the Shopkick SDK for the beacon to be picked up.

Source: 234 Android Applications Are Currently Using Ultrasonic Beacons to Track Users

The Burger King Hello Google ad is an example of this, except without advertiser feedback. Creepy.