Last week my SO was freaking out because she kept getting ads for things she had conversations about in person, but never looked up on her phone. A few days ago I asked her why we had a plastic bag from Games Workshop (we've never shopped there and the bag came from a Craigslist purchase we had made). Her phone was sitting on the table, and a few hours later she showed me an ad for Games Workshop on her news feed. This was on Android, and the Messenger app includes permission to use the mic at any time.

It might be audio being always on, or it might be more than that. They aren't above it.