Fly below radar
Fly below radar (English) — To conduct yourself in a manner, a way that does not attract attention to you. A person who is quiet and occupied would be flying below radar at work and such activities.
In the literal meaning it is to fly your aircraft below the altitude that radar is effectively operating. You are masked then by ground clutter, trees, buildings, mountains, etc..
Fly below radar — оставаться в тени; не высовываться; не привлекать к себе внимания