sly vs under the radar

sly

adj
  • Done with, and marked by, artful and dexterous secrecy; subtle 

  • Light or delicate; slight; thin. 

  • Artfully cunning; secretly mischievous; wily. 

  • Dexterous in performing an action, so as to escape notice 

adv
  • Slyly. 

under the radar

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

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