Under __, Idiom for Unnoticed, Undetected

This page is the CodyCross answer for the clue below from The Netherlands, Group 2112, Puzzle 3 (Zwolle and Enschede).

Question is: Under __, Idiom for Unnoticed, Undetected

and the answer is: THERADAR

Word Meaning:

THE — Used before a noun phrase, including a simple noun

Definitions from Wiktionary.