This happened to a friend of mine while travelling back from the USA on a business trip.
Selected for a search at Dulles after check in as 'something' showed up when baggage was screened. Taken to private area where suitcase was screened again test showed positive for 'explosives' - only a couple of people present at this time to take details. Suitcase opened and searched - dog handler (&dog) arrives, more details taken, dog handler states that the case should have been left closed as the dog was distracted by other things in suitcase (laundry etc) More people turn up, same details taken etc. My mate had a good idea what was causing the problem and pointed out a film bag. This eventually resolved the problem but at no point did anybody actually open the film bag to check what was actually inside!!