Years ago when transporting silver bullion from Melbourne to Sydney via Air I was pulled aside in Sydney into an interview room where it was explained to me that they could not distinguish via X-ray the difference between Silver bars and certain types of explosives.They appeared the same.Once unpacked and examined they were satisfied and allowed me to travel on.
That was my experiance and a recount of it, i make no conclusions otherwise as to their logic or reason.
Same!