sub rosa vs under the radar

sub rosa

noun
  • Covert surveillance video used to catch workers’ compensation applicants and show that they are in fact not injured. 

adv
  • In secret or covertly; privately, confidentially. 

adj
  • Carried out secretly or confidentially. 

under the radar

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

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