Curtis said her group is more concerned with the way the data is kept. "Police are not supposed to be keeping files on people who are not breaking the law," she said.
Valcarcel said police agencies typically keep the information for up to a year before removing it from the system.
On the outing in Jessup, the trooper drove his vehicle onto the parking lot at a light rail station. The constant drone of innocuous beeps stopped and the word "registration" flashed on the computer screen, followed by a series of faster beeps.
"What we have here is an alarm for a possible hit," Valcarcel said as he read the tag on the vehicle involved and compared it to the data on the computer screen. "It tells me that it's a partial read, it's not a match, so the system will reject it."
A few seconds later, there is another series of beeps, followed by the words, "revoked registration".
Again, Valcarcel compares the data on the screen with the tag on the car in question.
"It is a match," he said. "If the person were in the vehicle, we could take enforcement action if the operator was the owner. Since the vehicle is parked, we're not able to do anything with it at this time."
In the course of a 45-minute drive that included stops at Arundel Mills mall and along Route 295, his license plate reader scanned nearly 400 tags. There were a handful of "hits," mostly for expired registrations or suspensions because of overdue emission tests.
Last month, the trooper recovered a stolen car with the help of the reader. The driver was detained and arrested. "It was an unlucky Friday the 13th for him," Valcarcel said.