Bella shows up a bit later. "So if you have a spare quartz, you could try the same divination on me and see if there is magic 'about or on' me to the same degree, try to localize it?"
He fetches a spare quartz. He paces and chants.
A dim shadow forms on the surface of the water, then dissipates. The quartz doesn't react at all.
"...That... tentatively suggests you haven't had any exceptionally powerful magic worked on you recently," says Giles.
He tries it. Vague shapes under the surface of the water, almost but not quite forming the semblance of a human face.
"I'm hesitant to draw any firm conclusions. I don't even know if this sort of divination is able to notice the aftermath of a wish or similar effect, and I can't rule out that that shadow might have been its way of notifying us that your memories were recently altered... but in that case, there's no explanation for why Soph was so, er, vibrant."
"I could haul Charlie here, he'd have more memories altered than you if that's what's going on, if that's likely to affect the strength of the result..."
"The divination I'm using is very vague - I wouldn't know how to begin to refine it toward questions like 'has this person had their memories altered recently'; at the moment it's more along the lines of 'tell me one important thing about this person's nature or history', and it isn't even very good at doing that informatively. More results might just confuse the matter further."
"She took it, like, not well, but nobody yelled or threw anything? We're not telling Charlie yet."
"Or Renée but she's less of an immediate factor." Pause. "She seems nice. Soph does, I mean."
Sometime after sundown, the vampire twins show up and Giles is tired enough to welcome them into the library with hardly a raised eyebrow. They turn out to make good research assistants.