In Fate/Stay Night, in the 2nd Day of the Prologue, Rin takes Archer to the Fuyuki Park where the 4th Holy Grail War ended. While they are there, Rin's Command Spell acts up and because Archer can't sense any Servant, they figure out that another Master is spying on Rin.
Just using logic, I could assume it's not...
... Caster, as while she is the Master of Assassin, she is also a Servant and Archer sensed no Servants.
... Bazett, as Kotomine had told Rin that 6 of the 7 Masters are ready. While it's revealed in Fate/Hollow Ataraxia that she was Lancer's original Master, at this point, I could assume he had already attacked and stolen her Command Spells
... Shinji, as the day before Rin didn't detect the Bounded Field at the school, but the day after she does, which means while she was away from school, Shinji and/or Rider would have been setting it up then.
... Shirou, as he wasn't a Master at that point.
So that would leave Ilya or Kotomine. However, I can't be sure if Sakura had created the Book of the False Attendant at this point, or if Kotomine was actually doing his own recon, so I am wondering if there is any source indicating which Master was spying on Rin and Archer at this point?