under the radar vs undercover

under the radar

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

undercover

verb
  • To provide too little coverage. 

adj
  • Performed or happening in secret. 

  • Employed or engaged in spying or secret investigation. 

noun
  • A person who works undercover. 

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