There are many ways for law enforcement agents to track your behavior nowadays, but among the most pervasive — and perhaps surreptitious — are license plate readers. The Wall Street Journal this week published an extensive report on the technology, which was once under the purview of police and other public officials, but has since spread into the private sector, as well. Studies suggest that more than a third of the nation's police departments now deploy license plate readers, capturing millions of images and adding them to ever-growing databases. Declining prices have also encouraged some startups to begin building their own equally massive databases, though the practice has certainly given rise to legal controversy.