Rita Skeeter is a journalist for the Daily Prophet and an unregistered Animagus who would often turn into a beetle and snoop around to learn people’s secrets for her articles. This is never mentioned in the movies, but there are speculations of it being hinted at in The Goblet Of Fire when Skeeter conducts an interview with Harry and as she shoves him into a broom cupboard, she says “you should feel right at home, then,” possibly meaning that she knows Harry used to live in a cupboard under the stairs. The only way she could have known that is by listening to anyone who knew about it and mentioned the fact.