It takes a bit before Frisk notices Shiro out of the corner of their eye. They pause and turn, watching him with a calm, oddly flat expression before the smile faintly and wave to him, beckoning him over. They won't call out--that would be forcing him a bit too hard, probably, and he seems like he might just want to be alone right now. But Frisk still wants to offer the invitation, should he want some company.