Describing an action that cannot be detected. The term comes from aviation during World War II, where a plane is flying too low to be detected by the enemy’s radar. It soon was transferred to other situations, as in “I don’t want anyone to know I’m looking for another job, so keep my applications under the radar.” John Lescroart used it in A Plague of Secrets (2010), “Now I know . . . it hasn’t been the subject of much police concern so far . . . So up to now you’re all under the radar.”