Conan is already in the cover page of each and every DC comic. Why would he bother whether he needs to have a keyhole?
Gin is one of the high rankings...Gosho might want to save him for the last few volumes.
Borboun haven't been formally introduced yet. We still have no idea who he is.... and you're already expecting someone we don't know YET to have a keyhole?
And if shiho turns out at the keyhole, the black organisation might be able to track her once they see the book...jk