Update: The "mysterious" plane over Denver was indeed an E-6B Mercury, TACAMO - a mobile command platform used by the US Navy - the extra bumps are for communication equipment. While it's unknown exactly what it was doing, it's not at all mysterious. They fly practice and testing missions all the time. The only reason people noticed this one was that it left a nice contrail that showed the curved path it was taking.
DENVER -- Thousands saw a plane circling the Denver metro area late Wednesday morning, questioning where it may have come from and what it may be doing. Officials at a host of federal organizations had few answers.
Denver7 began tracking a flight named IRON99 as it traversed from California over the pacific. It traveled over the Rocky Mountains and eventually over the Denver metro area.
Public Information Officers at several Air Force Bases were able to confirm that such a flight maneuver suggests the plane is indeed a military plane, and may have refueled while in the air over Denver.
Officials at Buckley Air Force Base say it never checked in with towers on the base, and did not land at the base, despite flying over the base's airspace.
Denver International Airport officials also confirmed no such plane landed at the airport, and their towers wouldn't have come into contact with a plane uninterested in landing at the airport.
Further research into the plane's title of IRON99 turned up bases in the northeastern part of the nation that frequently give out the title of IRON to their military flights, however the title is usually reserved for T-38 aircraft, which are distinctly black.
When looking into IRON99, it appeared the flight had only communicated with the U.S. Army, and had no public communications available, as many commercial flights often do.