The answer to this question can be found starting in chapter 15 and continuing into chapter 16. Readers have likely been suspicious of the man in the yellow suit for quite some time, but chapter 15 really shows readers how nefarious of a person he really is. He absolutely doesn't care for the Foster family or Winnie's well-being. They are all simply tools to get him the land and the Tucks' secret. Chapter 15 sees him blackmailing the Foster family into giving him the wood in exchange for him saying where Winnie is.
And then the man in the yellow suit sat forward. His long face took on a hard expression. "Now, I don't have to spell things out for people like yourselves. Some types one comes across can't seem to cut their way through any problem, and that does make things difficult. But you, I don't have to explain the situation to you. I've got what you want, and you've got what I want. Of course, you might find that child without me, but . . . you might not find her in time. So: I want the wood and you want the child. It's a trade. A simple, clear-cut trade."
The man in the yellow suit then says he will go with the constable to get Winnie. Chapter 16 begins, and readers are treated to a wonderful conversation between the constable and the man in the yellow suit. It's a great conversation because we get to see the slow and fat constable annoy the man in the yellow suit to no end with his unceasing desire to go slow and steady and talk the entire time. Finally, the man in the yellow suit can't take it any longer. He tells the constable that he'll go up ahead and scout the place out because he is supposedly worried about the child. This is partly a lie. He doesn't intend to wait for the constable. He wants to hurry to get the Tucks' secret, but he also wants to hurry just to get away from the annoying constable.
The constable waited for more, but there was no more. His sour mood returned. He frowned and shook the ashes from his cigar. "Say," he said. "You're kind of a close-lipped feller, ain't you?"
The man in the yellow suit narrowed his eyes. His mouth, above the thin gray beard, twitched with annoyance. "Look here," he said tightly. "Would you mind if I rode on ahead? I'm worried about that child. I'll tell you how to get there, and I'll go on ahead and keep watch."