What call sign does Marine One use when the President is not on board?
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.