You've been in the high ranks of the knights of favonius for quite a while, hiding undercover in their ranks by being Lisa's assistant

Either by reading or practicing your elemental skills, time passed by for you, until a meeting was called in the acting grandmasters office

“... It comes as a tiny surprise, but I have reason to suspect a vampire among our ranks”

Jean says before pulling out a small file from under her desk...

It was quite a serious meeting considering Kaeya, Lisa, Barbara, Noelle, Eula, Albedo, and amber were there

Though it wasn't an exciting revelation, considering that vampire was you...how did they figure out so fast