https://www.techradar.com/news/research-claims-some-mobile-apps-are-secretly-recording-your-screen

It’s a conspiracy theory that’s been nurtured for a long time: someone mentions taking a holiday in the vicinity of their phone and the next thing you know, they’re being shown ads about flights and hotel accommodation on Facebook. 

People have believed that their phones have secretly been listening to them to gather data for targeted advertising, building a myth that a bunch of computer science academics at Northeastern University in Boston, Massachusetts, decided to look into.