under the radar
在監視或注意之外，隱秘地進行或發生
口語·表達隱秘行動
英文釋義
to go unnoticed or undetected; to operate or exist without attracting attention or scrutiny
例句
He kept his new project under the radar until it was ready to launch.
典故來源
源自軍事術語，指飛機飛行高度低于雷達探測范圍，后引申為不被注意或察覺的行為。