I know several people who have had the same experience. They hadn't searched for anything on the internet nor emailed or DM'd about it, was completely verbal. Am going to guess that the Google voice-activated app was responsible - it would hear something, then run with it all on its own.
Has to be something like that. As others have pointed out, the phone has to be listening all the time so it can hear you say, “Hey, Google,” or whatever.