So he was on duty "investigating a string of burglaries in the city" and was "in plain clothing", and driving an "unmarked police vehicle"?
So, essentially he was off duty?
These white people think a lot of people are just dumb. A city has many burglaries a day, so anyone can say there are a "string of burglaries" happening in a city on any given day. Those burglaries aren't necessarily connected with one another. The cop was off duty, wearing regular clothing, and driving his own car when he decided that Corey, a Black man, was suspicious being on the side of the road.