Saber was walking down a corridor in an area a long way from the lobby, when he suddenly stopped, and turned to look at something. He was staring in the direction of the lobby where the chief and his officers were fighting, but he had no way of knowing that.
“What’s the matter?” Ayaka asked.
Saber narrowed his eyes slightly, and answered:
“...I sense the presence of a monster.”
“A monster?”
“...Yes. It’s an old story.” There was a hint of sadness in his expression, rare for a man who usually exuded a wild air. “It was during a war. Monsters forced their way into a battle between me and my rival, slaughtering men of both camps. I sense a similar presence now.”
“I don’t really get it, but does that mean a monster got summoned as a Heroic Spirit?"
“No, I don’t think so. It’s not a Heroic Spirit. I don’t know whether or not they can even go to the Throne in the first place.”
Saber, experiencing a sense of foreboding, determined to be more wary of his surroundings and get Ayaka outside as fast as he could. He recalled those monsters’ peculiarities as he began to walk, and continued:
“To make a long story short... in your culture, they would probably be called ‘vampires.’”

Fate/Strange Fake: Volume 2, Chapter 3 ("Day 1, Early Dawn")