not entirely sure "despondent" is the right term for that face
Well, Nioshi's Koishi rarely has more than the one expression, so you need to pay closer attention to the tone of it. Her face is definitely shaded here, so I'd say she's despondent here. Otherwise, yeah, the permanent smile does make it hard to say she's feeling much of anything.
Koishi, where are youuu?
Not Being There Is the Cliche
.... Ah...!When you get people-hunting scenes like this......you just have to look where they absolutely won't be, don't you.