Spoilers comment
Wait if the MC got the 003 thing on his neck, does this probably means that other humans were on that world before?
Hmm, they are probably in formaldehyde jars aren't they-
Well, I suppose that with the information so far we can't prove or disprove that statement.
Regardless of it, you are right, there may be other ones hiding, but we won't know until more info goes up, besides, the creator of this game seems to like to put subtle stuff in the art and writing, so maybe we may find some subtle info lol.