Is that legal?

The other night I was driving home and I decided to take H street NE to Benning Rd. I pulled behind a group of cars waiting on the light to change from red to green. Of course, if you’re not familiar with that area of town, it’s currently undergoing major construction due to the Trolley system that’s going to be running through the H Street Corridor. Anyway, one DC’s finest patrol cars pulled up beside me and I could see into the their patrol car. The officer on the passenger side was typing in something on the laptop in the car. I realized that the set of alphanumeric characters the officer was inputting was the license plate number of the car in front of them. After he or she, for some reason the gender of the officer slips my mind, inputted the license plate number of the car directly in front of them, the screen did not show anything, at least from my viewpoint.  I started thinking to myself they must be following this car and was checking to see if the car was stolen.  The officer then proceeded to input another set of alphanumeric characters. I then realized that this input was the license plate number of the car in front of me.  I was like wow.  Jake (slang term for police officers) is just riding around checking people’s license plate numbers. They probably ran my license plate number as they passed my car. I wonder if this type of activity is common across the city or is it just practiced in majority black neighborhoods. What do you all think?