UFO sightings in SD

So, I'm driving on the freeway, near the marine base, and I see something silver in the sky. We're stuck in really bad traffic, so I pull over by the side of the road. My girlfriend asks me what the heck am I doing, so I point out the tiny silver disk in the sky. I didn't have my camera with me, so I couldn't take pictures, but from what I could see, it was a small silver disk, really far away, just a dot in the sky. It was moving way to slow to be a jet, neither commercial or military, and it wasn't moving or looking like a military helicopter. After a minute or two, it just vanished. Kinda wierd, huh? Does anyone have any sort of idea of what it could be?