G2Tactics' Mobile License Plate Reader is a hand held thing that at first glance looks like a laser or radar, but is really a plate reader. They also have something called the Bootfinder that is similar.

http://www.g2tactics.com/BootFinderV1.wmv is a video of the Bootfinder in action.

GLAVID plate finder is a similar system, and they show this one mounted on the roof of a patrol vehicle. At first glance you might think it's some sort of IR NightVision device.

Some of these are used in New Haven CT, Bridgeport CT, and Arlington County VA