The CFD vehicle which Sean McCaffrey (Joe Anderson) drives is numbered "4-2-2", which would be the vehicle number & radio call sign of the Deputy Director of Fire Operations for the Office of Emergency Management & Communications. As an OFI Investigator, his vehicle/call sign should be 4-6-7, 4-6-8, or 4-6-9.
Fire Department vehicles and apparatus are depicted with strobe bars on their roofs with red & blue lights. In actuality, the Chicago Fire Department uses light bars with red & white lights, with the exception of the light at the far right (passenger side) of the vehicle, which is green in color.
When Agent Johnson is talking to Sean after buying a hot dog from a sidewalk vendor, there are no signs that the grill is actually cooking anything. There should be smoke, sizzling meat, etc., but there's nothing.
The CFD vehicle driven by Brian McCaffrey (William Baldwin) is numbered "2-2-2", which would be the vehicle number & radio call sign of the Deputy District Chief of District 2. As the Commanding Fire Marshall, his vehicle/call sign should be 4-6-0.
Two of the OFI Investigators get out of a vehicle numbered "2-2-4"; this would be the vehicle number/call sign of the Deputy District Chief of District 4. As OFI Investigators, their vehicle/call sign would be 4-6-7, 4-6-8, or 4-6-9.