Describing these mass surveillance devices as "license plate readers" is a deliberate tactic to deceive people about their actual use and capability. Flock cameras record everything, not just license plates. They record the faces and bodies of the driver and the passengers that are visible through the windows. They record and analyze all the little dings, bumper stickers and other unique elements of a vehicle above and beyond license plates. They are regularly deployed and used in areas like footpaths where there is no vehicle traffic at all.
They are also able to read / capture things like stickers on your car, facial recog, etc.
Just to add, some can identify Bluetooth signals, and I wouldn’t be surprised if identification via direct TPMS (tire pressure) is added eventually.