Completely unseen, carrying that quantity of blood? If it were a swarm taking small amounts, there'd be more bites. Noticably more. The only possibility would be injecting something into them that thins out the blood and is removed through some 'natural', unnoticed process. Bowel movements, most likely.

At any rate, I've had them brought to the city hospital. Those specialists will certainly look into such a possibility, too.