I wish I knew what those apps were. Or what signs I can look for. I generally don't have many "garbage" apps on my phone, but I've always suspected it with words with friends--Yes, my gf have been playing that game together since we started dating 5 years ago. Every so often an Ad will come on and the volume all of a sudden adjusts itself from mute to about 33%. Its distributing that an advertisement can have that power.
edit: Hey wait, let me read the article--just poured my first morning cup of coffee and reading HN.
I think it's an app specifically installed for the purpose. I just asked him what it was called, he said it was an Arbitron(now owned by Nielson) device. I googled Arbitron device and found this: https://en.m.wikipedia.org/wiki/Portable_People_Meter
Note that it doesn't use ultrasound, it embeds information in the normal audio in a way that humans are not likely to perceive (psychoacoustic masking).
edit: Hey wait, let me read the article--just poured my first morning cup of coffee and reading HN.