Joking aside, what do police have that scans license plates when they drive and alerts if any match to existing warrants?
That technology is in use here but most cities have banned them due to privacy issues.
ALPR cameras are mounted on the exterior of vehicles, and utilize a high power IR beam that cause the reflective coating on license plates to glow - increasing the contrast so the software can conduct the OCR properly.
The privacy issue is complicated. Technically, anything that can be viewed from the public space/domain is fair game to anyone with a camera. There are numerous court cases, and DOJ memos that explicitly permit all forms of recording in public on the basis of 1st Amendment protections. While it might not seem like a 'free speech' issue to have a camera in public, the fact that anything that is recorded
might be used in publication or in some other form of speech (i.e. blog, YT, etc) means restricting your right to record is considered "prior restraint" - in essence, keeping you from getting your data for your speech issue. Ultimately, photography in public is not a crime.
So, if the police (or repo guys) want to spend their time photographing every license plate they see, they are legally justified.
Where it gets a lot more complicated is when they run every license plate through a database without probable cause or, at a minimum reasonable articulable suspicion (RAS). In Florida, for example, and officer can't just arbitrarily run someone's tag through the Driver and Vehicle Information Database (DAVID) without cause. Officers that do, violate state law, and can be prosecuted. The principle behind the restriction is protection from unreasonable searches and seizures (4th Amendment).
Having every visible license plate (and therefore vehicle owner) treated as though they might be a criminal, and their record searched without probable cause or RAS is arguably a violation of their 4th Amendment rights. Put another way, it would be like police stopping everyone exiting a mall and searching their pockets, wallets, and purses to see if they can find any contraband. Or, police entering every home on a given street (without warrant) to see if they can find any criminal activity. Sadly with TSA, and now more random 'checkpoints', we've become accustomed to surrendering more and more of our privacy, liberties, and freedoms - all in exchange for some perceived safety/security.
Separately, if a private/corporate entity invests in the technology, develops their own database of vehicles subject to repossession, and then scans every vehicle they encounter, I DO NOT take issue with that. Namely, they are not government, and their actions do not involve criminal databases, nor do they have the authority to arrest, detain, or otherwise bring down the justice system on your head.
Sorry for the political slant - but it's important to understand why these cameras (in the hands of government) are the camel's nose under the tent.