Can a cop carry a firearm wherever?

Not necessarily. For example, police officers are required to check their firearms when flying, unless they meet certain specific criteria (basically, they must be performing a job that specifically requires them to be armed during the flight, or at least at some point between baggage check and baggage claim, such as prisoner transport; a police officer travelling on vacation must check his firearm the same as any other passenger). They also must have filed a specific travel authorization form, and completed a (short) course specifically related to flying armed.