Loosing a Registered Pouch is very hard unless it was stolen. Everytime the pouch touches someones hand, they have to sign for it. It starts with the clerk that took it in, then the driver who transports this, etc.
Usually the package goes into a lock pouch which goes into another locked pouch.
All I can say is that once the Supervisor informs the Postmaster that the register is missing, the Postmaster has to inform the Postal Inspection Service. Someone's job is on the line.
This will be found, but when is the question.