"Missing" seems to imply that they used to, or should, know where they are. It seems unsurprising that they don't have definitive location of most of their currency, it is traded by individuals with little to no tracking other than when it touches a bank.
It seems like all of these numbers are just estimations, so it's more like they can't guess, and get people to confirm, where the rest of these notes are or how they are being "used".
I understand that but they have estimations for known unknowns, this is claiming that the whereabouts of most banknotes are unknown unknowns. That's interesting because it very clearly shows how poorly trackable cash is.
It seems like all of these numbers are just estimations, so it's more like they can't guess, and get people to confirm, where the rest of these notes are or how they are being "used".