How would the pilot of Marine One identify himself to air traffic control without the President on board? Does it have an N-number? If anyone knows, I would like a specific call sign/N-number of any of the Sea Kings. In particular, how would a Sea King pilot flying alone identify themselves to air traffic control?
Marine one is the call sign of any of the (300+) United States Marine Corps aircraft carrying the President of the United States. So, if he's not on board, the aircraft reverts back to its original radio call sign.
In case you are asking about the Marine Helicopter Squadron One, you can find the ID from helis.com. HMX-1 uses the call sign 'NIGHTHAWK' usually, but this is prone to change depending on the circumstances. From the site:
In Korea, call signs changed on a regular basis; during the Vietnam War, squadrons might use the same call sign for more than a year. Today, squadrons deploying to a combat zone,i.e. Iraq and Afghanistan, are assigned a "theater" call sign for use during that deployment only.
US military aircraft use mission callsigns, which can change with every flight, not tail numbers. Therefore, it is impossible to provide a comprehensive list for what a particular aircraft might use.
That said, squadrons often perform the same mission repeatedly, which means using the same callsign prefix, e.g. "Nighthawk" for the squadron you're asking about. That prefix is usually followed by a mission number, similar to an airline flight number.
However, some missions are unique and therefore do not need a number, and some missions (e.g. Air Force One and Marine One) always use the same number. The only hard rule is that there should never be two aircraft in the air at the same time with the same callsign.