Despite not being too far from the woods, Frisk is unaware of the night's disaster. Just far enough not to hear his sounds of pain, deep in their own dreams (maybe).

So the first time they notice anything is in the morning, after they've decided to work on their tree-climbing skills. Their meeting with the newer Chara made them realize how little they ever really tried doing that.

The beast made of bones might spot them staring down at him in surprise, perched fairly high in a mango tree.