[It's politeness rather than a lack of perception that's kept him from noticing these things earlier. You don't go noseying around in other people's business just like that, but there's only so much he can ignore and those furtive glances just slipped out of the mind-your-own-business category.
He follows the path of her gaze, quietly noting the similarities between the two girls.]
Someone you know? [Stupid question maybe, but how else is he supposed to approach this?]