covert vs under the radar

covert

noun
  • A covering. 

  • A disguise. 

  • A feather that covers the bases of flight feathers. 

  • Area of thick undergrowth where animals hide. 

  • A hiding place. 

adj
  • Under coverture. 

  • Secret, surreptitious, concealed. 

under the radar

prep
  • Without attracting notice; in an undetected or secretive manner. 

How often have the words covert and under the radar occurred in a corpus of books? (source: Google Ngram Viewer )