A helicopter is no different from anything else. And radar doesn't care whether you're in a helicopter or whatever.
All the radar cares about is whether you're reflecting energy back to its reception antenna.
A helicopter will do that, IF it's in line of sight to the antenna (and to the sending antenna, which may be somewhere else).
At 5m above the ground, a helicopter or indeed anything would have to be pretty close to the radar installation to do that, not only due to the curvature of the earth but the transmission angle of the radar installation, terrain features, etc..
For example even a low rise in the terrain, like an earthen berm protecting the radar installation, or some trees standing around it, a shack or house in the vicinity, would be enough to hide something from detection.
And yes, many radar installations can be set up to ignore things that don't have at least a certain speed and/or are below a certain altitude. This to prevent cluttering the displays with radar returns from birds, cars, motor cycles, and things like that. But that's a function of the display unit, not the radar receiver. It still gets all those returns, there's just a software filter between it and the screen that declutters the data for easier interpretation by the radar operator, who should have an option to tweak or even turn off that decluttering as needed.