Paging Hans Blix, please answer the red phone in the lobby.
There was a large underground military complex at Qiqihar in N. Manchuria. It is believed to be Imperial Army's chemical weapons' facility. The underground complex is still around in that city. Occasionally, local Chinese reportedly came in contact with buried chemical agents abandoned by Japanese, suffering from debilitating side-effects.
Maybe Japanese are looking for missing chemicals in the wrong place.