But we never left a "sterile" area -- from the time we left the aircraft on arrival at SEA, through immigration and the baggage hall and the Customs gantlet, we were secure and sterile and airside as anything. We'd been "sterilized" when we went airside at Heathrow ten hours prior. And when you exit Customs at the South Satellite you are still in a "sterile" zone until you get off the automated train back at the main terminal. The security folks we encountered were just re-sterilizing us for no apparent reason... (and this innovation is definitely post-9/11).

Now that you mention it, I've never understood the insistence on re-surrendering checked bags at the Satellite either... I must be terribly dull and slow.