Can a helicopter mask itself from radar?
Note that for advanced radar systems, moving helicopters might always be distinguishable from cars and other objects, due to the fact that helicopters have moving rotors.

While I have no information on military systems, I know the systems by robinradar can separate drones from birds by using doppler shift techniques.

Doing this on an actual, full-size helicopter without rotor guards should be a lot easier than on a drone.

This means avoiding detection by moving slow and staying low might not work, unless you can stay out of the line of sight of the radar system.