It takes her three steps to notice, and she only notices because accepting a Counter Guardian within herself boosted her base physical stats.

There is somebody else in this place. Somebody well hidden.

Her brain received a lot of information upon being possessed. She understands the concepts regarding Heroic Spirits, Servants, and the qualitative assessment of their abilities. She knows there is such a thing as an “Assassin-class Servant”, with the Skill “Presence Concealment”.

Maria quickly dismisses that knowledge. If this were in fact that Skill in effect, she would not be aware of a presence in the first place. If this were a Servant, it would register in her mind as an unmistakable “mark of mystery”.

She is just dealing with a mortal particularly skilled at skulking around.

Day 1 Evening 01 - When It Snows