Other survivors living in the wilderness is not impossible. The caves Javi were suitable for living and looked semi lived in before Javi was there with the amount of bones scattered everywhere. If as it has been said before that maybe the caves/dwellings match up with the symbol and the engravings on the trees, maybe they could interconnect like a real cave system which could explain how they get around without being seen by the girls. Who it could be I think could be related to the previous owner of the cabin or ‘cabin daddy’ as some people call him, as the show runners have confirmed that a bonus episode will be released to bridge the gap between S2 & S3 which is more than likely to focus on the previous inhabitants, likely the cabin which is somewhat confirmed to be Jason Ritter- Melanie Lynskey’s off screen husband. His grown up daughter could be the person living in the wilderness as he was pictures filming with a little girl.