At the end of VV1, there is a mention of evidence of recent human activity in the cave, in VV2 it's confirmed that you are sharing the island with another group that has a long history on the island,
in VV3 and 4 it's learned that certain former inhabitants are aware of the newcomers,
and in VV5 you find that there is yet another group,
and there was a history of a single tribe that splintered into two factions.
The eyes behind the boulder could be a member of either of these tribes, Biggles,
or one of the old ancestors.