Strange Small Craft with 4 Engine Jet

I was taking a walk here in north Denver yesterday (gorgeous weather, for once) and I noticed the tell-tale contrails of a 4-engine jet aircraft heading in my direction from the southeast. I assumed it would be a 747, but also considered an Airbus A-340 or a KC-135. As it seemed to fly almost directly over me, but slightly south of my position, I looked up again and the plane did indeed appear to be a 747, though there was still a slight bit of doubt. It did not at all look like a military transport (C-5 or C-17). It had a white belly and gracefully swept wings and looked like an airliner (747 or 707 based aircraft, the military uses both platforms as well). Each engine produced a contrail. It seemed though, that there was a free-flying object directly behind the right wing of the big jet, just in-board of the number 3 engine. From my vantage point a couple of miles below, it appeared to be moving with the same speed of the larger plane and I could not detect significant variations in distance between the two. It looked grey, certainly darker in color than the big jet. It did not appear to produce a contrail. I was only able to observe the strange craft (if indeed it was an aircraft) for 15-20 seconds before the pair had passed to the northwest and grew smaller.
I've been around planes bit and small, military and civilian, all my life and I fly a vintage plane as a hobby. I've seen countless jet airliners streaking across the sky, but never have I seen anything like this. Perhaps it was a wierd reflection and there was nothing there at all, but I believe I saw some object tucked in very close to that jet. Any ideas or similar sightings?

[edit on 12-2-2007 by Airknocker]

Could it have been a mid-air re-fueling?

I doubt it was a refueling. US tankers (except Navy carrier-based tankers and C-130's) normally use a flying boom to refuel, extending out from beneath the tail of the aircraft. Many European nations use a hose-and-drogue system that extends a hose behind the tanker's wing. If what I saw was an in-flight refueling, it was not a conventional method. The object seemed too close in to be using the hose-and-drogue method.

What you probably saw was the Honeywell 707 test aircraft with an instrument pack extended, thats been flying quite a bit recently around your area.

That could be what it was. From the ground, it looked as though the object was seperate from the airplane, but at that distance, it's hard to judge. After looking up some photos of the airplane you mention, I'm inclined to agree with your suggestion. The Honeywell testbed has a 5th engine mounted on the right side, forward of the wing. Perhaps what I saw was some sort of instrumentation, or some sort of contrail relic from the 5th engine.

Here's a link to photos of this strange aircraft.
www.airliners.net...

Sounds like the B707 used for transporting engine pods.

Shattered OUT...