Leaving the Maverick, looked in the sky, saw a white shape flying north maybe 500 feet in the air, maybe 20 feet diameter going fast

Working for a IF City department, leaving the Maverick on Sunnyside and S Yellowstone walking north to the truck with two other coworkers. I looked up and saw and object and said "what is that?" The other two looked up and saw what I was looking at. It was all white, and round. The underside appeared to be grey maybe due to shadow. The shape to me looked like a somewhat flattened ball. It was maybe 500 feet in the air and headed in a northward direction at a high rate of speed. We watched it until it traveled out of sight to the north. Maybe 10-15 seconds total. It did not make a sound. I immediately pulled out my phone and checked the Flight Radar app and there were no planes in the area. One guy I was with agreed it was a UAP. The other one is very skeptical. Doesn't think it is a UAP, but has no idea what it could have been. If it was that low, we should have heard something. If it was higher than I thought, it was going at an incredible speed. I did not think to try and video it at the time. I wanted to take it all in with my eyes. I don't care if I'm contacted by an investigator, but this is mainly for documentation.