What did Timothy suspect about the island that he didn't tell Phillip at first?
I believe that this question is asking about events in chapter seven. Phillip and Timothy have finally gotten themselves ashore on a small, deserted island. Timothy leaves to scout out the island and find a suitable location for shelter. When Timothy returns, he tells Phillip that the island is quite small, but it has some positives to it. Phillip is suspicious, though, that Timothy isn't giving him all of the information.
"You are worried about something, Timothy. Please tell me the truth. I'm old enough to know."
Timothy then confides to Phillip that they are likely in an area known as the "Devil's Mouth." It is an area of ocean and islands that is not frequented by ships. It is a dangerous area for ships because of the numerous sharp coral banks. Timothy says that it's a large "U-shaped ting." The small cay that they are on is one of many small cays that is contained within the ring of coral. The coral prevents ships from getting close, so Phillip and Timothy now have almost no chance of being spotted by a passing ship.