If I am correct, it is a LPR (License Plate Reader) device. LPR devices can "read" all of the license plates they "see". The data read is checked against a local database in the squad car (which is typically updates on at least a daily basis.
In addition to reading plates and checking a list, normally GPS data is incorporated in the system - to record the location where the license plate was seen..
This type of system is in use in the Minnepolis/Saint Paul metro area.
Yep, it's a license plate reader. Some mount them on the top of the car, but most are mounted on the rear quater panels.
They are used mainly for id'ing stolen and wanted vehicles; kind of replaced the old "hot sheet"