No matter what, the ones who perceived the presence of the strange sorcery weren't Waver and Rider alone.

The spell-like waves emitted from the surrounding of Mion River were close to the multiple aria of the ritual magecraft class, something which could not be activated without at least 10 people. Certainly, every magus in Fuyuki city―namely, all the masters participating in the Holy Grail War, would have sensed that at once.

- Fate/Zero: Act 10, Part 1