How has Vera laid out a trap for Mr. Nuttel?

Vera is able to lay out a trap for Mr. Nuttel because when she first meets him she ascertains that Nuttel doesn't know anyone in the neighborhood. Nuttel explains that his only connection is "my sister was staying here, at the rectory, you know, some four years ago, and she gave me letters of introduction to some of the people here" (Saki). After confirming that Nuttel knows almost nothing about Vera's aunt outside of her name and address, Vera is able to lay a trap for Nuttel.
Vera notes that Nuttel says his sister was in the neighborhood four years ago. Therefore, she makes up a tragedy that happened three years ago, after Nuttel or his sister could have had any knowledge of the neighborhood.
Vera indicates the open window to Nuttel and says that he must think it strange to have an open window in October. She then tells him that three years ago, her uncles went out shooting and were swallowed up by a bog, but the "bodies were never recovered" (Saki). She tells him, "Poor aunt always thinks that they will come back some day, they and the little brown spaniel that was lost with them, and walk in at that window just as they used to do. That is why the window is kept open every evening till it is quite dusk" (Saki). Vera plays the part and acts as though this story that she is telling saddens and disturbs her.
Nuttel believes Vera, and when the aunt appears and says that her husband and brothers have gone out shooting but should return for tea, Nuttel is increasingly alarmed that the aunt is not in her right mind. He believes Vera's story that the men died, and so he thinks that the aunt must be deranged. When Vera's uncle and her aunt's brothers do indeed arrive home, Nuttel has become so nervous that he believes that the men are ghosts, and he runs from the house as quickly as he can.