“...Inside her? Who is it?”
“That’s the question, now, isn’t it? They haven’t revealed themselves yet, so even Watcher isn’t sure. Of course, when it comes to someone that strong, that damned Watcher ought to be able to guess by their presence, but...”
At that point, he vanished, and the boy with the snake staff took up the conversation.
“She’s completely isolating her presence. She’s also utilizing her power in spite of that, so she must be remarkable. It isn’t just Watcher; even Enkidu, who possesses presence-detection abilities of the highest level, hasn’t noticed her existence, or that of Haruri’s Heroic Spirit.”

Fate/Strange Fake: Volume 4, Interlude ("The Parade of Treachery")