well... very few custom will search ppl leaving their country...
so it's the Japanese 海關 that missed them from entering into their country.
actually, i think most countries custom search people when leaving the country instead of entering?
im not sure about japan.
but when entering in hong kong, people can easily walk pass customs and towards the exit without them checking (unless you look really suspicious)
i might not sure though, correct me if im wrong. =P