Which type of test is more likely to be used, based on practical considerations such as cost, ease of use and so forth?
They were probably relying on that swipe test. If the package was in a remote area it is doubtful they would have moved it to a large machine like this one:
http://www.lifesafetysys.com/osb/itemdetails.cfm/ID/379
Here is where it can fail...Just suppose someone had fired a gun or handled one that had been fired...Like... for example, whatever agent was taking the swipes. Or suppose someone had been exposed to some other organonitrate..for example, wearing a transderm nitro patch!
The machine would see the swipe sample, and go bananas.