I walked outside to take my dog out. I noticed trails in the sky from airplanes, then noticed an object moving very fast following along one of the trails from the smaller end to the larger end, going from South to North. It was moving much faster than a plane usually does and it was almost blending into the background of the sky. I watched it for a few seconds before it disappeared. I always look at the sky when I'm outside and love watching the clouds etc, so I see a lot of airplanes and helicopters. This object seemed to be different from the normal every day traffic in the sky. I thought it was strange but didn't know where to report it or if I even should until I saw an article about a sighting in Denver that gave this sites information.