Police Taxi:

This car with multiple sirens and the casual pullovers and busts that vehicles of this type pull is on the low end of 'undercover.'

elmblog wants to know what makes it noticeable as a police vehicle. And I guess that's important information so:

• On the trunk lid, there is an extra black thing protruding about 4 inches. This is a police radio antenna or something like it. 
• The black trim on the side doors, door in this case, is rarely ever on a real New York City cab anymore.

Other features on the police car that are not shown in this picture:

• There is no partition, and while there are a few taxis without partitions, the front seats are made of cloth rather than the leather like material mandated by the taxi commission.
• Sometimes an on board shockproof laptop computer is mounted between the two front seats.
• The Police-cabs are often driven in groups of three very police looking people.
• And the most obvious reason, is sometimes that the car is parked on the same block as the police precinct and the bulb siren is sitting on the dashboard.  

UPDATE - 2-11-09...

Although the strange license plate number in the picture below doesn't match the roof number (medallion number), this has now been addressed. Now both the plate number and roof number match like a normal taxi's, but another taxiblogger, G.S. at: Cabs Are For Kissing, has mentioned in a much earlier post that the police cars that go undercover as taxicabs will most likely have the first two digits be 2W or 6Y. I have seen many police taxis and not one didn't follow his rule. So here are more updated identifying characteristics of the police cars in disguise:

• The medallion number will very likely start with either 2w or 6y
• More of these cars are carrying a fake advertisement on top. YES, believe it or not, and this ad is always in pink advertising a fake drug in soft white italics, for example PROGESTRA, yeah somebody has a sense of humor. rather than having a light up medallion number on these ad tops, they taped on a number with white tape, and may also have one or two cameras inside.
• I forgot to mention previously that all these cars are the Police Interceptor model. While most cabs around the country are used police cars, our yellow-cabs were purchased exclusively for the purpose of taxi driving and have an extended rear door, which the police cars do not have. The undercover police taxis do not have an extended rear passenger space.
• These cars do not function as taxis, they never pick up passengers, and they are not driven by licensed cab drivers. They do busts and pullovers just like other police cars.