Basically, there are 3 kinds of MBB, Tyvek MBB, AMBB and ANMBB, in the the market. The key factor of moisture barrier function of the bag is metal layer, like thickness of the foil layer, foil or metalized, etc., but thickness or material of other layers also can partially affect the quality of MBB. TMBB has no metal layer, but will be used for less moisture sensitive device and this bag can also be a good choice for foods packing, which requires clean and vacuum properties. So I'm assuming the TMBB has a higher MVTR than normal MBB thus making it not worth using as a canine sweep is the most likely cause of detection. That's assuming they haven't implemented more stringent screening as as matter of course.