Nobody keeps strict tabs on the children as a general rule, but when Cheryl can't find Alberto one night quite late (possibly coming back from the cave with someone), she gets frantic. This is shortly after his arroyo episode.
He's under the chaise lounge in Despina's hovel. He goes in while she is still at the fire circle, just at dusk. Sleeping on the floor is not a hardship to him. Despina's bed is hung with mosquito netting, and the draping falls down on each side of Alberto's hiding place. He likes it under there. He likes the smell of her hovel.