Jack explained to Reika about a technique called 'spirit form' that completely erases her from view.

"Is it different from what you used last night?"

Jack answered, "That's right.

"What we used yesterday was the 'presence concealment' skill of the Assassin class. We didn't disappear, we just erased our presence. We can't carry anything while in Spirit Form."

"I couldn't see you at all. Did you really just erase your presence?"

"That's just the sort of creature we killers are. We're confident that not even a Servant can ever detect us as long as it's nighttime."

- Fate/Apocrypha: Unbirth