Something under the radar goes unnoticed. If you're coming in past curfew, you'd better stay under the radar to avoid getting in trouble.
Radar is used to locate objects by bouncing radio waves off them. It's often used in the military to detect enemy aircraft. However, by flying at very low altitudes, pilots can manage to evade detection. This maneuver led to the idiom flying under the radar, which eventually shortened to under the radar, meaning "unnoticed" or "undetected."