Most commercial speakers (e.g. computer/smartphone) can emit ultrasounds and microphones (e.g. of smartphones) capture them.
A TV ad may contain an ultrasound tag and a smartphone app could activate your smartphone microphone "to listen" on standby.
Similarly, when you visit an online store your browser could activate the emission of an ultrasound.
New Scientist article: "Your home’s online gadgets could be hacked by ultrasound" https://www.newscientist.com/…/2110762-your-homes-online-g…/
"The staff of the Federal Trade Commission has issued warning letters to app developers who have installed a piece of software that can monitor a device’s microphone to listen for audio signals that are embedded in television advertisements."
@informationbookcom facebook post
Posted 2016-12-31 (cf this page)
In 2016, the US Federal Trade Commission took action on ultrasound device tracking originally identified by the German hacking club Chaos Computer Club who is the organizer of Europe's biggest hacking conference, the Chaos Communication Congress. Lecture by Vasilios Mavroudis on 2016-12-29 at this year's 33rd edition of the conference #33c3: http://goo.gl/KVZmB8
A lecture by Vasilios Mavroudis