under the radar
在监视或注意之外，隐秘地进行或发生
口语·表达隐秘行动
英文释义
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.
典故来源
源自军事术语，指飞机飞行高度低于雷达探测范围，后引申为不被注意或察觉的行为。