behind the scenes vs under one’s hat

behind the scenes

prep
  • behind the scenery and stage area; backstage; among the actors; during the production or rehearsal 

  • In secret; out of public view. 

adj
  • Being or working out in secret or out of public view. 

  • Divulging or reporting hidden workings. 

under one’s hat

How often have the words behind the scenes and under one’s hat occurred in a corpus of books? (source: Google Ngram Viewer )