Hear me out on my theory, [spoiler ][/the reason why one or more knockers were missed by both Fushi and black hood. We were never told that all of Hayase's descendants can only bear one child and how the knocker is inherited. So what if, one mother could have three children? I'm assuming that they think only one of them must've inherited the knocker given the bulge in their left/right arm, but what if a piece of that knocker also exists in the other two children, therefore migrating to future generations without ever being known by either black hood/fushi/defense corps because it's so miniscule... And the knocker that at first was so noticeable became smaller each generation because it was being split up between the children. Also, as the knocker gets more split up the less likely the mother will die, which is why the first few generations only had one child, because the knocker was too big... Which is why we don't see such an obvious bulge in Mizuha's arm and why her mother is still alive. Perhaps this time Fushi will have to fight the entire human race, idk, I've had this idea since Mizuha first started scratching her arm, and I thought oh there's a tiny knocker there and maybe she has inhuman capabilities.
because the one that was in Hayase's descendants probably]