There's lots of possible reasons that aren't nefarious. Could be training of pilot or crew (I once watched a passenger aircraft orbitting the base I was stationed at for most of a day. Turned out it was commercial pilot training and check rides). Could be test flights of equipment and systems for airworthiness certfication. Could be DoT testing navigation aids, etc. If the government wanted to surveil Seattle residents for some reason, I expect the NSA could come up with something a little more subtle.