I'd lie, like everyone else - the rather pointless question attempts to determine if someone has packed some form of bomb inside your luggage without your knowledge. In reality, the poor sod who would act as the "mule" will have been told by their "lover" some cock and bull story about their being something special in the bag for them and to answer the question as I have suggested.
It is another example of "stable doors" and "bolting horses."
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