If mmWave sensor can penetrate an object?

Hi

Theoretically FMCW can penetrate through structures. It can detect objects in rain/snow and hence moisture shouldnt be a problem. It should also be able to detect 4cm objects easily.

But practically, it depends on lot of physical parameters, like reflectivity of the intruding structure, how far the sensor is placed, reflectivity of the Object etc.
It also depend on the Antenna design and the emitted waveform. Another thing to consider is whether the object is moving or stationary. How strong is the clutter in the environment also affects the detection capability.

In short, It should be able to detect in your usecase but unless more specific details are available, it cannot be accessed.