In Tatarigoroshi-Hen, after Keiichi kills Satoko's uncle, he becomes paranoid when he realizes two things:
A) the body is not where he buried it
B) Satoko's behaviour implies he's still around, alive
The explanation for A) is no doubt the same that happened for Rena here in Tsumihoroboshi-Hen. They hid the bodies in the same place as Keiichi killed Teppei in Tatarigoroshi-Hen; the forest. According to Mion the bodies they hid with Rena in Tsumihoroboshi-Hen would be in danger of being discovered if they deforested the area, so she used her connections to have them moved because of her friends, and the Hinamizawa philosophy of helping fellow villagers out, right? Well no doubt the same thing happened in Tatarigoroshi-Hen. Rena got paranoid in Tsumihoroboshi-Hen because she thought Mion was against her when she moved the bodies without telling. For Keiichi in Tatarigoroshi-Hen, it caused him to panic - why isn't the body where he hid it? Probably because Mion did the same and moved it. Obviously Mion and Rena knew what he did. He didn't hide it well and you can tell they knew when they covered for him by saying he was at the Cotton Drifting Festival. That brings us to B). The doppleganger Keiichi fears of him that is hanging around isn't real, that was just the cover from Mion and Rena to say he was there. He really wasn't, obviously. But Keiichi is becoming delusional at this point so combined with the missing body and how it seems like Teppei is still alive, he wonders if maybe he has a doppleganger. And as for why Teppei is 'alive' it's only because Satoko herself is delusional and is convinced he's still around abusing her when he's really not. But all these things combine together and result in the Keiichi of the later parts in Tatarigoroshi-Hen.
Tsumihoroboshi-Hen has similar elements of what happened here, even though it's more directly the answer arc to Onikakushi-Hen. But we know that for the mystery of where the bodies went, Mion had the Sonozaki family hide them, just like she probably did for Keiichi in Tatarigoroshi-Hen. And we know that the whole 'doppleganger' issue is still here in this chapter, and all the others, with Takano - whose body should have already been dead by the time of the festival. We know that's because she wasn't really dead and used an already dead body to burn and pretend was her, solving the mystery of how she was at the festival when she should have been dead. So as of that point all the mysteries surrounding missing bodies and dopplegangers, which are a main cause of Keiichi's descent into paranoia in Tatarigoroshi-Hen, can be explained logically.
So here's my question...is it ever explained why Rena is 'found' elsewhere when she's hiding from the guys in the junkyard? Rena's paranoia here is like Keiichi's in Tatarigoroshi-Hen; he thought there was a doppleganger of himself somehow lurking around, but we know he was crazy (he thought he could kill people by wishing it :P ). For Rena, it's belief in Takano's notes, about aliens being the true cause of the Hinamizawa Syndrome. You can specifically see this in
this from the TIPS thread about a portion the Anime didn't cover in great detail. So that's why Rena thinks the aliens made a clone to 'replace' her because they intend to get rid of her since she has Takano's notebooks and knows 'the truth'. But what is the real reason that Rena is spotted elsewhere?! Is that ever explained? Is it just like in Tatarigoroshi-Hen, and maybe Mion and co. are covering for Rena and trying to mislead the police? But how do they know she's really at the junkyard, just a good guess because they know she hangs out there? Though in the post I just linked to, Rika knew to find her there, so she could have had a part in that. But I don't think this is ever directly explained.
Basically the question is, who is the Rena that is supposedly found in the quarry, when the real Rena is hiding in the junkyard?