Which Master was spying on Rin?
Caster is the only viable person to be able to use such an underhanded method, via her Far Sight (i.e., her crystal ball). 

Shinji is incompetent to the point where he can't even provide his servant with prana, much less do basic magecraft. 

Bazett is no longer in the picture by this time do to a certain priest. 

Illya has no reason to, because she has Berserker. 

It's not Kirei because he is the mediator, so the identities of all the masters would be known to him sooner or later, because all the masters are expected to report in to him. 

Only assasin is capable of spying,  but the Assassin of this Grail War is confined to a shrine on a mountain...