Once I started looking for the plastic plate covers I was actually shocked how common they are. Of course enforcement is so lax these days many people seem to be using a paper temporary plate that they printed out. No word on how many of those are even real, I can't even read the numbers on them through the window.
In my town some of the police cars have darkly tinted plate covers.
Did you see the one which used an electromagnet to hold fake leaves in place? If they got pulled over, they could push a button which would allow the leaves to fall off.