Not that long ago, I had an experience with this[1]. I'm currently running a test with my old phone to confirm that it is listening to me. I have an old android phone, with the microphone disabled (tape on the input)

Every once in a while, I will have the tape removed and say a few keywords, then disable it. My hope is to see if I will receive any advertising in with these niche keywords. It's only been a week so far, but I will update the post if there is anything.

(Note I have create a new google account just for it.)

[1]: http://idiallo.com/blog/be-mindful-with-ad-targeting (disclaimer: my blog)