I think Satori was last spotted at the Myouren Temple, in chapter 33. She left to do... something.
It's possible she's the one talking to Reimu via the Yin-Yang Orb:
- When she left, it was right after she said that having Reimu breaking into the underground guns blazing was actually the better option compared to the alternative, keeping the underground sealed off. (It's possible she knew that Yuugi would side with Mizuchi eventually and break her out, knowing her backstory, and knowing Yuugi. Hence her suggestion that keeping the underground sealed would be impossible.)
- Whoever is talking to Reimu through the orbs - they're knowledgeable about vengeful spirits and the underground.
- In chapter 33 talking to Byakuren, Satori suggests it would be a good thing for Reimu to go be "the game protagonist she is". This matches with what the person behind the Yin-Yang Orb tells her to do in this chapter: "Be as flashy as possible". The person behind the Yin Yang Orb also refers to Utsuho as a "final boss". More video game terminology. It seems Satori is intending to use Reimu as bait.
There are reasons it's probably
not Satori, too. Mainly that we haven't seen her, and when you aren't shown who somebody is, it's never the first person you think of. So I'm going to name the last person you'd think of - Tamatsukuri Misumaru. She crafted the orbs, which suggests she has some ties to the Hakurei Shrine as well. Unfortunately, there's a non-zero chance that in addition to everybody else, ZUN's forgotten she exists.
Now what might Marisa be doing? We last saw her in cahoots with Reimu about her extermination spree. Ichirin namedropping her seems to be the story telling us to wonder about it.