User blog comment:Zippertrain85/The reasons why Serana is not from the First Era./@comment-9369193-20130510195601

Every vampire movie I've ever seen the vampires seem to sense other vampires by how old

or from whom their bloodline comes, Any vampire should be able to recognize Serana's blood

and therefore recognize her instantly because only Harkon or her mother would be the only

other similar "sents" even if they had never seen her before.