Do scanners and pat downs take into consideration that the explosive material may not be something hidden at all? It could be the suicide bomber himself. That is, chemicals that are impregnated into the skin as a lotion, woven into the fabrics of clothing, etc. It might not be what's inside a container but the container itself. Chemical "sniffers" might be more useful than visual scanners.
It reminds me of this old story:
"One day a guard stopped a worker who was walking out of the dockyard gates pushing a wheelbarrow with a suspicious looking package in it. The guard opened the package and found it contained nothing but some old bits of rubbish, sawdust and floor-sweepings.
The next day he stopped the same worker who was again pushing a wheelbarrow containing a suspicious looking package. Once more it contained nothing of any value.
The same thing happened several days in a row, until the guard finally said, 'OK, I give up. I know you are up to something, but I just can't tell what. Please, I promise not to arrest you, but put me out of my misery; tell me what you are stealing.'
'Wheelbarrows, 'smiled the worker, 'I'm stealing wheelbarrows.' "
