In Fate/Zero, it seems Caster was able to find Saber far too easily. I can understand how he found her during her and Lancer's fight, as Lancer had "sent an invitation" to all servants and Masters. Rider even noted that the other servants were sneaking around watching the fight.
Before the attack on the Einzbern Castle, he intercepted Saber and Irisviel while they were driving (he was waiting ahead of them on the road), and he was able to quickly locate the Einzbern castle despite the fact that the Einzbern's wouldn't make their base of operations so easy to find, since I remember that in Fate/stay night, Ilya or Rin noted that the forest around the castle messes with people's senses and makes it hard for them to escape from it while being hunted down by Beserker.
I can understand how Medea was able to track everyone in Fate/stay night as she set up her workshop on the leyline nexus at Ryuudou Temple and was manipulating all of Fuyuki's leylines from that point (not to mention her abilities make her seem like a god to modern magi, so Einzbern would be child's play). I also assume Rin's knowledge of the castle came from the fact that by Fate/stay night, she was the second owner of Fuyuki, and as such, she would be aware of the Einzbern and Matou's locations, along with every other Magus workshops legally operating in Fuyuki.
However, in Fate/Zero, Caster had neither of these things, and both him and his Master didn't operate like proper participants in the Holy Grail War. So how did he keep finding Saber?