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(CNN) -- A passenger who landed at Tokyo's Narita airport over the weekend has ended up with a surprise souvenir courtesy of customs officials -- a package of cannabis.

Sniffer dogs failed to find the cannabis after it had been slipped into a passenger's bag.

A customs official hid the package in a suitcase belonging to a passenger arriving from Hong Kong as part of an exercise for sniffer dogs on Sunday, Reuters.com reported.

However, staff then lost track of the drugs and suitcase during the exercise, a spokeswoman for Tokyo customs said.

Customs regulations specify that a training suitcase be used for such exercises, but the official had used passengers' suitcases for similar purposes in the past, domestic media reported.

Tokyo customs has asked anyone who finds the package to return it.

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