Lots of license plates these days have high contrast between the letters and the background. These license plates are often usually covered in retroreflectors such that the high contrast is accentuated with any kind of light. A strong IR light source from an IR camera can probably read most clean license plates driving by without being obvious to the people being viewed. Remember that light follows the inverse square law, so if you're far away you'll probably need a pretty bright IR source to get a good high contrast read from a far away license plate, along with pretty good optics/sensor to reliably read the plate.

Both of the comments below are correct. In addition, the system I worked on had a strobe that flashed when the camera took a shot. If you zoom in to just the plate it's actually pretty difficult to differentiate a day shot from a night shot.

Ohio at least requires that there be a light that illuminates the liscense plate whenever the headlights are turned on, so no need for special night vision systems