Why????

Maybe something happened in the area and they were just collecting info. I agree, however, that they should have told you they were done. You hate to have to ask, but you could.
I am not in law enforcement in any way. However, I always try to support those who are and don't know what you'll do when they approach you.

Your car looks great against the mountains. I don't know much about photography, but I know cars look great right about a half hour before sunset.