What is the situational irony in "The Open Window" by Saki?

Situationalis when something happens that is
completely opposite to what the audience expects. When the neurotic Framton Nuttel arrives at
the Sappletons' country home to rest his nerves, Vera meets him at the door and immediately
assesses that he is a vulnerable, gullible man. Vera proceeds to tell a haunting tale about why
her aunt leaves her large French window open. Vera tells Framton that three years ago, Mr.
Sappleton and his two younger brothers-in-law went hunting and left through the French window.
Tragically, the three men were lost in a bog that day and never returned. Ever since they
disappeared, Mrs. Sappleton leaves her French window open in anticipation of their return.
Suddenly, Framton sees the three men walking toward the window and panics. He works himself into
a panic and ends up fleeing the Sappleton home without saying goodbye. The situational irony
occurs when Framton...