Frost forms in a freezer mainly due to moist air getting in. The moisture then freezes out onto the cold surfaces. Other than opening the door to get items out of the freezer, the most likely way for moisture to get in is a poor door seal. Try the "dollar bill" test. Put a bill or some other piece of paper in the seal area and close the door. If you can pull the paper out easily, the seal is likely leaking in that area. Do this test all around to door, as the seal may be good in some places and bad in others.
Just read your post again and noticed that you left the freezer on for a week with nothing in it and didn't get frost. If you left the door closed all that time, or opened it much less than normal your frost problem is probably related to opening the door frequently when you normally use the freezer. The very moist air in the northwest is a contributor to the problem.