this is not a taxi. it will not pick you up.
i saw one today (for the first time ever) and yes, it freaked me out.
Here's additional info I gathered from the blog of cab driver...
1. the police are not usually in uniform, and there is always more than one person in the car
2. if the medallion number (the four digits in the rooflight used to identify the car) begins with 6Y or 2W its a cop car
3. the usual yellow strip on cab doors is black on these undercover cabs