Plane Spotters: Help Us Identify This Warplane's Mission

On Sunday 25 January 2009, what is probably a US Navy C-9B Skytrain logistics aircraft was photographed at Shannon airport in neutral Ireland, at about 11 am. Even though it had no obvious military markings, it was being guarded by an Irish army security patrol. The registration number it is obliged to display was incomplete. The only number it displayed was 0050. As far as we can establish its full aircraft registration number is 160050.