Just FYI, I used RCWL-0516 in the past. I did not find any issue with its range. The 10cm that you see on yours is rather low. If you have another one, you may try to give it a try it could be a bad copy.
Another Radar sensor that I have used is below. I probably use the same concept as RCWL-0516 with different working frequency. I used it here for my DIY Arrival Sensor.
The above module and RCWL-0516 does not detect static human presence. They are PIR like sensors where there must be motion for it to detect presence.
There are other type of presence detection using Radar. They are given nickname mmWave. It is actually just much more advance signal processing where it can detect static human presence. These are more expensive. They are in the range of $30+. In addition, their typical current consumptions are 60mA or more since they require MCU or DSP to do signal processing. It is not suitable for battery operated project.
The basic technique is FMCW. I do not have FMCW based module at the moment to play around. I will get one eventually. I am curious how well the signal processing on these FMCW modules.
