From my partial moonspeak knowledge, Okuu is flying home when she sees someone that she thinks it's Koishi. Her thoughts are so "loud" that Satori can hear them. Satori hurries after her sister.
It's an underground forest. Notice that there are cave walls over it instead of the sky.
Eh?S-Satori-sama!!?Aangh...just now I felt......and I'd swear she said, "Koishi".I knew it!So what's up?Well, it's Okuu, but...Eh!?But since it's her, some solid proof would...Oh, there she is.Eh!?SplashWhewWhat the heck is going on!!?And what are you doing?You should hurry and go, right?!B-but......I can't tell from here if it's true or not...Idiot!.... You said you believe in her, right?Orin! Get a change of clothes ready!!!Aye...ShaplashObviously!Her expression changed and she took off.She is worried sick!At any rate......if she really has come back......there's no happier event.DASHShe vanished!!!All the way in the middle of the forest...When I get back, I'll give Satori-sama some boiled eggs too!FlapOver here!!?One, two, three...I've got seven, so...Huh?FlapflapflapflapFlap...half for each of us, so Satori-sama gets...three!No, wait, two?One will be enough!Hm?It's a person.She vanished!!?Ah!KOISHI- SAMA!...she saw Koishi or something...Ah! She vanished!!?STABKOISHI-SAMA! IT'S KOISHI-SAMA!!!...Ma'am!!!Looks like!That she'll come home.So obviously she's there!...an incredibly......strong thought......from very far away.That was Okuu's voice......See, I knew it.