At the end of chapter 3, Simon goes off by himself into the the trees and other flora of the island. The narration says that he "...walked with an accustomed tread through the fruit trees..." which indicates that he's been in this area before. In fact, because he is very observant and detail oriented, he sees fruit that others don't see and he gets it for the other boys, especially the little boys. After having satisfied all those clamoring for the fruit, Simon goes further into the jungle. He looks around to see if anyone, particularly Jack or Ralph, is watching. When he is satisifed that no one is watching him, he worms his way under a bunch of vines into the center of this area where he is hidden from view. Since he knows exactly where to go, he clearly has been here before. The narration implies to the reader that Simon goes to this place when he wants to be alone. It is in this same secret place that Simon speaks to the Lord of the Flies in chapter 8. This is the place Simon goes to when he wants to contemplate. It's also possible that when Simon feels a seizure coming on, he goes here to be alone.