When Jacob comes out of the cairn, what does he think is happening?

Jacob's made the fateful decision to return to the house, which he's going to explore. At first, it's quite a fascinating experience, discovering all those dusty old photos, toys, and various bits of bric-a-brac. But then things start getting more than a little scary. First of all, he hears a strange voice; then he looks up from the spooky basement to see half a dozen kids whose faces he recognizes from the photos staring down at him. Jacob wants to reach out to these strange apparitions, but they instantly scatter in all directions. Jacob runs outside after them, but they've gone.
However, Jacob hears a noise and turns round to see a girl in a white dress. He follows her into a cairn, or large mound of stones. As he walks along inside the cairn's dark, dank interior, Jacob tries to reassure the girl that he means no harm. But he eventually realizes that there never was any girl; he must have been hallucinating.
Dejected and humiliated, Jacob emerges blinking into the light. As he squints through his split fingers, he barely recognizes the world around him. Everything's the same as it was before, except that now the whole scene is bathed in brilliant golden sunlight. The sky is blue, and there's no sign of the fog that Jacob has always associated with this part of the island. And it's warm too. The weather sure changes fast around here, thinks Jacob.