It doesn't always matter what's inside. At Seattle, you can see what they're doing since it's right out in the open: for many of the domestic flights, they're opening and swabbing all bags that are flagged by the computer, not X-raying. Not enough X-ray machines to do them all, so they employ people to just open, swab, check, and close. (people are cheaper and take up less space than X-ray machines)