Impossible to stop really. All the terrorists need to do is what the drug couriers routinely do. I can see stool checks before boarding next.
I could see them using some sort of x ray machine for all passengers or selected passengers, The x ray machines would probably catch something like that. My guess is they could probably take x rays of individuals secretly when they walk through the metal detectors or maybe newer redesigned metal detectors with x ray capabilities that some one would be looking at on the other side of the wall.