Granny is suspicious of the man’s yellow suit.
Winnie is outside looking for fireflies when the man in the yellow suit comes along at sunset and talks to her. You can see why Granny would be suspicious of this.
The man in the yellow suit asks Winnie how long she has lived in the house outside the wood, and she says “forever.” He finds the answer interesting. Of course, she does not mean the answer literally, but it is ironic given that he is looking for the Tucks, who could possibly live in a place forever, though they would never come out and tell someone!
Winnie finds the man in the yellow suit both charming and oddly disturbing.
But Winnie, though she was half charmed, was suddenly reminded of the stiff black ribbons they had hung on the door of the cottage for her grandfather's funeral. She frowned and looked at the man more closely. But his smile seemed perfectly all right, quite agreeable and friendly. (Ch. 4)
Granny asks Winnie who she is talking to. She tells her it is a man, and she comes to investigate. When the man in the yellow suit sees Granny, he stops to talk to her and asks her if she is fit.
"And why shouldn't I be fit?" she retorted, peering at him through the fading light. His yellow suit seemed to surprise her, and she squinted suspiciously. (Ch. 4)
Granny is suspicious because he is in a suit, and a yellow one at that. It is flashy and preposterous. She is also suspicious because he is talking to her granddaughter. She asks who he is, and who he is looking for. He answers neither question, but says she probably knows everyone who comes and goes."
She says she doesn’t, and doesn’t “stand outside in the dark discussing such a thing with strangers” (Ch. 4). It is at this point that they hear the music box.
"Wait!" said the man at the gate. He had stiffened, and his voice was eager. "You've heard that music before, you say?" (Ch. 4)
Granny insists it is elves, but the man in the yellow suit is excited. Even though Granny has not directly answered his question, he knows that he has his answer. The Tucks are nearby.
The man in the yellow suit is the villain of the story. He is the one who wants to profit from the Tuck family’s discovery, without really thinking about the consequences of living forever as they have. He represents the materialistic viewpoint that usually comes with the fountain of youth narrative—exploiting it for financial gain.