Date: 201302
Local time of day: 1201-1800
Location: ZDV.ARTCC, CO
Aircraft: Learjet 45
Flight plan: IFR
Flight phase: Descent
Airspace: Class A ZDV
ATC / advisory: Center ZDV
Second aircraft: Military
Reporter organization: Government
Reporter function: Enroute
Reporter qualification: Air Traffic Control Fully Certified
Human factors: Confusion
Anomaly: ATC Issue All Types
Detector: Person Air Traffic Control
Result: General None Reported / Taken
Contributing factors: ATC Equipment / Nav Facility / Buildings
Primary problem: ATC Equipment / Nav Facility / Buildings

Narrative: I was working Denver Center ZDV Sector 24 combined with Sector 23 FL270 and up. A Lear 45 was westbound to LAS at FL430 and would soon be descended to FL320 as per our Letter of Agreement with ZLA for LAS landing traffic. The Lear 45 pilot asked about traffic at his 11 o'clock and 20 miles 5;000 FT below flying in tight circles. I had no traffic in that location or anybody holding or circling. I identified to the Lear one aircraft at his 12 o'clock and two others in his 9 o'clock (one of which was at FL380). I also 'quick looked' the low altitude (FL260 and below) below my sector and found no traffic there. During this event; the Lear reported both 'painting' this mystery aircraft as well as identifying it visually. The pilot reported that it looked like an F22 or F35. I had the two aircraft which were south of the Lear look also; however neither saw anything visually nor on their TCAS. This caused a fairly casual session on sector to become difficult quite quickly. I still had to work the rest of the sector and aircraft. Now I was fairly busy trying to locate and identify an aircraft in my sector that were not displaying or talking to me. In addition; I needed to descend this Lear for his landing at LAS through the altitude of this mysterious craft. It made for an uncomfortable and potentially unsafe situation. The pilot said he thought this other ship looked like a small fighter jet such as a F22 or F35. I understand that the military might want to conduct 'top secret' missions of flying through our airspace and don't want us to know. I can only assume that if that is what they were doing; they should be staying out of the way. However; an airliner and controller stumbling onto that flight is a situation that obviously can happen. If the military doesn't want to share what they are doing; we should at least have some type of procedures; rules; and training on what to do if a pilot suddenly asks about a mysterious aircraft that doesn't appear to be there. Perhaps a way to quickly confirm that this 'surprise' aircraft was indeed under it's own special program so that we don't have to worry about it anymore. That would differ from still trying to identify someone flying where they shouldn't be. Obviously; safety is the concern. No collisions between unknown aircraft; no overloading of a sector trying to identify an unknown situation or aircraft when others are in full knowledge of the situation.




Synopsis: LR45 crew informed ZDV Controller of traffic ahead and below that appeared to be conducting tight circling maneuvers; the Controller did not observe any traffic on the RADAR display.