Later:

Integra: Who could have done this?

Alucard: I sensed no one sneaking onto the grounds, Master.

And it’s a largely unknown method of subduing a vampire. We’re dealing with an expert.

Integra: A very strong, or subtle, expert.

Walter: Sir…

Integra: What is it, Walter?

Walter: If I may…before we panic, there is a far simpler explanation which we ought to look into.