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.
Actually, the government (NNSA, https://nnsa.energy.gov/) trains for that on a pretty regular basis.
They scan cities for known sources of radiation (hospitals, certain types of industries, natural sources) and map it all out. They'll also go back and conduct drills using those maps to see if they can locate planted radiation sources.
The lack of sense of humor here is stunning. Other comments here are deep in conspiracy-land. Forgive me for contributing a differing, humorous viewpoint.
