Right now, to find the enemy hiding in this castle, Kayneth must be searching all of the rooms on the first floor one by one. At the moment Kiritsugu is located at the innermost part of the second floor. If he acts immediately, he should have enough time to choose a place advantageous for him to face the enemy.

Kiritsugu analyzed the plan of the castle in his head as he walked out of his room and towards the door – his footsteps stopped.

A drop of mercury, like a thread of cobweb, hung in the door’s keyhole. Although it was only a tiny bit of mercury, Kiritsugu could still see it drooping toward the ground as it left a silver trail on the surface of the door.

The drop of mercury suddenly stopped moving downwards the moment Kiritsugu saw it. Then, it retraced its tracks, retreated back through the keyhole as if it was alive, and disappeared.

"… So that’s what it is; an automated search."

- Fate/Zero: Act 6, Part 4