I've become totally superstitious (convinced, even) that phones are listening to more than just ultrasound. There have been too many uncanny situations where youtube or google have recommended results that were far too specific to the conversation happening moments before. For example, one time I was telling my girlfriend about fourier transformations, something she'd never heard of before. She typed "Fo" into her phone. "Fourier transormation" it suggested. Maybe this was due to the IP address we were connected through. That alone is creepy enough as it is. Sometimes I wonder if it isn't something more though.

Facebook messenger and google voice search are my number one suspects. But moreso, all it takes is a single person in a room full of phones to have installed an app that's using the microphone unethically, and suddenly everyone's conversation is being turned into marketing/spy data. Anybody who's connected somehow to that location is going to be statistically associated ("implicated") with this data.