The bodies were drained of blood, and then thrown in the ocean (the Thames?) whereafter some bodies were found, and others where not. The only thing incontrovertible was the blood pool left behind in the domicile of the victim.
She might not have been found?
It's just a question where you get 8 pints other blood without the lady dying?
Stockpiling a surplus or a massive circular transfusion?
(Or is Moriarty behind the people who invented making blood?)