UFO Sighting occurred when observing the sky on 3/24/18 over Auburn WA. I noted what I first thought to be a large bird traveling at moderate speed in a westerly direction. As the UFO was headed west, a commercial aircraft was leaving KSEA (SeaTac) on a southern flight path. I attempted to point out the "bird" to a friend who did not locate it. At that point it was rather small in the sky and was basically a dark UFO. The sun was low to the SW. I noted a very bright reflection that seemed much large in scale then the sizeof the UFO. I can completely rule out an aircraft, as the direction of flight would have been an airspace violation. Once I witnessed the unexpected shimmer, I lost sight of it.
I estimate that the UFO Sighting traveled from the Auburn valley to I-5 in less than 60 seconds. There was a distinct cloud ceiling, which is was under. Conditions at KSEA are as follows: Conditions at: KSEA (SEATTLE/METRO , WA, US) observed 0153 UTC 25 March 2018 Temperature: 7.2°C (45°F) Winds: from the WSW (250 degrees) at 6 MPH (5 knots; 2.6 m/s) Visibility: 10 or more miles (16+ km) Ceiling: 12000 feet AGL Clouds: few clouds at 3000 feet AGL scattered clouds at 6000 feet AGL broken clouds at 12000 feet AGL broken clouds at 20000 feet AGL The traverse of the sky seemed to match that of a large raptor at approx 1000 feet altitude. The time it took to gain distance, etc. This is what I thought I was seeing, until UFO brightly reflected the sun for a brief moment. It it were at 5000 feet, UFO was traveling at approx. 250 knots. At 10,000, triple that.
I report this UFO Sighting in case others may report a similar or related incident. source nuforc.org