(Many thanks to 'You Bi Ji Yue', 'OppaiIsKing', 'Wufeng Haotian', 'A Person Who Loves Lolis', 'Diying', and 'ZHUJIAOLONG' for their tips!)
Because Caster was considered the weakest class, she was at a disadvantage against the other Servants. Thus, Caster Servants generally could not confront other Servants head-on and had to consider other ways to win.
And outsmarting one's opponent was clearly a crucial part of that.
Caster had not suddenly come to attack without thinking anything through.
In order not to alert Noah and the others, Caster had deliberately chosen broad daylight, when she was least likely to be suspected of making a move. She had also done her utmost to conceal her presence, intending to deal with Sakura Matou first.
In Caster's plan, the ideal outcome was naturally to seize Sakura Matou's Command Seals and thereby control Saber, making Saber part of her own fighting force.
With Saber, a Servant from what could be called the strongest class, plus the strongest magus capable of supporting her with all kinds of magecraft, Caster was confident that she could even handle the most troublesome Berserker.
And when Caster discovered that Rider was not at Noah's side, her confidence grew even stronger.
Unfortunately, that plan had been declared a failure from the very beginning.
Aside from Rin Tohsaka, Noah with his sensory abilities, Saber with her Instinct skill, and Archer with his Mind's Eye skill were all extremely sensitive to abnormalities.
Even though Caster had done her best to hide her presence and had chosen to act in broad daylight, she had still been discovered immediately, causing her plan to fail.
Of course, Caster did not know that this plan had never had any chance of succeeding from the start.
The reason was that Noah's scabbard was still hidden inside Sakura Matou's