To second what WPD said, you shouldn't have a queen excluder during the winter. During the winters, the bees will cluster to keep the hive warm, and move around the hive consuming the honey for energy. If the excluder is in place, the queen will quickly be left behind the "ball" of bees, and freeze. It's possible that with no queen, they decided to abscond rather than try to build queen cells during the winter.
Also, I would very, very strongly recommend that you not use foundation (or at least use the smallest cell foundation you can find, currently 4.9mm) if you're going organic. The standard foundation has been proven to be too large, making it difficult for bees to fight off varroa. My strong suspicion is that foundation could be the smoking gun that caused CCD in the first place, but that's another debate all together.
The movement of the excluder is most likely due to a mouse or some other critter that loves to nest in the warm honeybee hives over the winter.
Best of luck with your next try! Since you're in Seattle, you may want to try a swarm lure to see if you can't get some local stock to start your next hive with. I don't believe AHB is a problem there. Bringing in packaged bees may guarantee you what breed you get, but they'll have to climatize to your area before they'll be stable. This can take several generations.