Why does the man in the yellow suit go to the Fosters' house? What do the constable's remarks seem to foreshadow, or predict, for the Tucks?

The story's opening prologue tells readers that the man in the yellow suit stopped at the Foster's house because he was looking for someone. We are not told who at this point, and the text tells readers that the man in the yellow suit did not say who he was looking for. The next time we hear about this stranger is in chapter 4. Winnie is playing in the front yard, and the man in the yellow suit approaches her. The man and Winnie begin talking to each other, and he asks how long Winnie has lived there. Winnie says "forever," and the man's interest is quite piqued.

"Perhaps. In a bit," said the man. "But I'd like to talk to you first. Have you and your family lived here long?"
"Oh, yes," said Winnie. "We've lived here forever."
"Forever," the man echoed thoughtfully.

A few lines later the man admits that he is looking for a family. Winnie's grandmother comes out and the conversation more or less repeats itself with her. Then the group hears some music coming from the nearby wood. Winnie thinks it sounds like a music box, but the grandmother thinks it is elves. The man in the yellow suit does not comment on what the sound could be, but he leaves quite happy while whistling the tune.

The man came to himself and sighed. His expression was one of intense satisfaction. He put on his hat, and in the moonlight his long fingers were graceful and very white. Then he turned and disappeared down the shadowy road, and as he went he whistled, very softly, the tinkling little melody from the wood.

He is looking for the family that he heard about growing up. It is a special family that supposedly does not grow any older. That family is the Tuck family.