Sakura and Shirou
Sakura gave the girl a sheepish smile.
"Well, my magic is naturally difficult to track, so I thought a normal familiar would be fine", she replied. "Anyway, at least we've learnt something about her abilities, and she wasn't doing anything particularly unusual as far as I could tell, so I don't think it's a big problem."
"And, yes, of course you can take a look. It should be fine, though, I've had to do this before when spying on more sensitive targets", she added.
Then, Sakura got to work, focussing on creating a familiar which would leak as little mana as possible, using the Matou trait of absorption that had been carved into her body by Zouken.
"Thank you for offering to assist us. That woman seems like she could be dangerous, we need to find out more about her", he said. "How close do you need to be to listen to what she's saying? If we can listen from a safe distance, it would be very helpful."
As she spoke, a small shadow familiar was forming on Sakura's hand, emitting barely any magical signature at all. All of the magical energy it emitted was being absorbed into Sakura's shadow dimension, using the absorbtion magic of the Matou to her benefit.
"Here you go", she said with a smile. "How's this?"