Beijing has tons of black, license-plate less cars just driving around, breaking laws. Nobody seems to know who they are - they're some kind of security. They have these very deep, official sounding horns they honk all the time. They don't usually egregiously break laws, but they'll do things like go through a red light if they see nobody is coming, or park in the middle of the road, and they always assume they have the right of way. Oh, and they love to drive on the highway shoulder while everyone else is stuck in traffic on the ring roads.
They have blacked out windows, but if you do get to see them, the drivers are usually young / middle aged, pudgy, and not in uniform (unless you count tight polyester turtleneck / stripey shirt & manbag as a uniform).
In addition to the unmarked cars, there is another annoying group of cars you'll see - ones with military plates. Their license plates are marked wit ha special red character (army, navy, air force), which makes them completely above traffic laws. So regular police can't even pull them over. It's fun dealing with these inviolable entities during day to day interactions! The guys driving them never have uniforms on, either.
Luckily China' effective anti-corruption policies have prevented these valuable license plates from falling into the hands of non-military officers.
There are hierarchies in who is under the law. These high-status people driving around, constantly flouting the law also prevents any common feeling for a general right, fair way to behave from developing. Everyone can see that the people at the top are rude and inconsiderate, and that the story about "rule of law" is a joke - it's "rule by law".
Policemen can't by default enforce the law, either - they have to check the plate first.
From a safety standpoint, it is dangerous to have people driving around without license plates, because if they hit someone and drive away, there's basically nothing that can be done to figure out who they were. And if you are hit by a military car, it takes guts to do anything.
Sensitive areas in Beijing have even more black cars, just sitting around without license plates or any other form of ID on the car. In places like this, how do they actually know that the approaching black Audi is valid? Common people definitely can't just park wherever they want in a security area, yet there doesn't seem to be any security checkpoints for these guys, and they don't have plates, and have blacked out windows - so what's to stop someone else from just driving up? It seems like even internally they must have some kind of system. Lots of other systems full of powerful people still have ID requirements.
So, I think there probably actually is some kind of identifier on these vehicles - and probably something that can be detected through a security camera. Infrared reflective strips or something on the roof & sides? Or is it just a particular model of car that can't be gotten except by the right people?