It's likely an undercover/plainclothes cop. The white sweatband (I called it an arm band before because I couldn't think of the word) is a common identifier used by plainclothes police officers. It's called color of the day (ie it's not always white). There's a wikipedia article on it actually.

The white armband on his left upper arm is a definite sign of an undercover cop. It's called color of the day or something, basically it's to signal to uniformed officers that he is an undercover cop.