Tecmo kept Other M so secret Koei didn't know it existed
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Nintendo, Team Ninja, and Tecmo were so secretive regarding the development of Metroid: Other M, that Koei had no idea it existed for some time.
Even when the two companies merged, Tecmo was not allowed to disclose the project per the arrangement with Nintendo.
No word on when the game became common knowledge.
More over on Siliconera.