Around and around and around Frisk goes. Where will they stop? Sans doesn't know.

Sans does a thorough check of the inside of the castle - and spends a while standing thoughtfully outside a certain tower cell - before he thinks to check outside. He doesn't want to go looking for the kid's hideout again. But he also really, really wants to see for himself that Frisk is okay. So, as he steps outside, he's hoping to meet them along the way.

It's anyone's guess who will see who first. Frisk might catch sight of the floating blasters to mark Sans' position. But even if Sans sees them first, he can guess that surprising them might not be the best move right now. So, albeit with a sigh of reluctance, he starts waling over.