I've seen these a few times before, but never caught one on camera until a few days ago. It looks just like a regular yellow cab, right up until the officers within set the strobes ablaze and are WHOOP-WHOOP-ing the sirens.
I've heard that you can pick them out from the rest because the number on the roof light doesn't match the license plate, but I've not been able to verify this.
How many of these roam the streets? What's their mission? My best guess is to catch other cabs in cab-like offenses; Taxi crime division(?).