under the radar

Reader's　Digest 3月号の記事 "10 MINUTE LIFE FIXES" からの引用です。

Keep Your Fly Closed
Realize a little too late that the zipper on your favorite jeans has gone faulty and won't stay up? Slip a key ring off your key chain and scurry away to the bathroom. Attach the pull to the ring as you would a key, close the zipper, and then hang the key ring over your pants button. The zipper will stay up on the teeth and under the radar until you can get the pants to a tailor.

"fly" がいわゆる社会の窓の意味であることは知っていますが、"under the radar" の意味が分かりません。辞書を見ます。

・Merriam-Webster English Dictionary: not getting attention : unnoticed: He tried to stay under the radar as he went about his business.

・Wiktionary: Without attracting notice; in an undetected or secretive manner.: Institutional change has not happened. But it's done quietly under the radar at lots of places.
Etymology: An allusion to flying an airplane at an altitude too low for radar detection.

・Urban Dictionary: when someone is in a situation where very little or no attention is drawn to them: somehow they managed to stay under the radar for years while skimming all that cash from the bank.
なるほど、語源が分かったので直ぐに覚えられそう。