She may not know the reasoning behind that expression, but she can understand how even something like a piece of jewelry could hold profound emotional significance for someone. It could be something close to Kanaya's heart, and at the very least it's obviously something she doesn't want to talk about, so Ahiru follows her lead and only responds to what Kanaya says, not what she's clearly omitting.

"Oh, those are good points! So what are you thinking of using the Department for now?"