That's why I was thinking it was depicting how the original Lost Children (i.e. Tofo and the parents who left the kids behind)came to that part of the island. The third person is in the water as well but he/she is not yet falling over the waterfall.

The only way it could depict the other two villagers would be if it were a prediction, which could possibly happen: recall the dream that one villager had which showed how to solve one of the puzzles in VV1. However, this is more doubtful, especially seeing as there are three villagers in the picture and not simply two.

There must be a way (somewhere) to get back up to the cave anyhow and not only because of the paintings. Recall in VV1 the eyes which peered from behind the boulder and the fresh food found in the cave. Unless there is another hidden exit from the cave which hasn't yet been revealed, the unknown person had to make it back from the second part of the island.

Edit (as this just came to me):

Oh, another thing I found curious was the bowl of fresh berries. The fish I could see, as there was fishing in both VV1 and VV2.

However, there are no berries in VV2, only in VV1. So where did the person who was hiding in the cave get those berries? Could VV3 have a berry bush as well? Or was someone somehow getting food in there?

Anybody else have any ideas?