The same thing has happened to me twice, apparently in my case when my shoes had been in the bag and they had left faint traces of fertilizer from the grass. I would imagine that they are trained not to discuss the procedures for testing with you so that you cannot gain information that could be used to circumvent the system. It would appear to me that the system worked in this case in the way it was supposed to, in that you weren't carrying explosives and were allowed to go through.. Can't you just forget about it?