Your app has a map. You didn't really think about it when you turned it on — it seemed handy for seeing where your friends were. But it works both ways: it shows other people exactly where you are, in real time, down to the street. Today a kid you barely know mentioned, casually, that they saw you were "at the leisure centre earlier." You never told them that. They just watched the little dot with your face on it move around the map all afternoon. It's a strange feeling, knowing people can see where you are without you ever telling them.