"Why do you have my mother's scarf?" (16: Snapshots.36)
The bright red scarf Lindsey sees on Len Fenerman's desk clearly belongs to her mother. When Lindsey sees it, and asks Len about it, she realizes the truth of the affair. Now, we notice that Shakespeare's tragic Othello is referenced quite a few times in the novel. A similar handkerchief is a huge symbol in that play.
Here's a crash course in Othello: The handkerchief, Othello's first gift to his wife Desdemona, is a symbol of her fidelity so long as it's in her possession, but a symbol of her supposed infidelity when the evil Iago plants it on Cassio. The catch is, Desdemona isn't unfaithful. So, the handkerchief becomes a symbol of the type of jealousy that leads Othello to murder his wife.
Although Lindsey and Buckley become jealous of Abigail's life outside of them, jealousy isn't a quality exhibited by Jack, and he never learns of the affair. So, the scarf is transformed in this novel. It becomes a symbol of the fact that Abigail made her own choices about whom to have sex with, and isn't required to die as a result. This is also a sharp contrast to the fate of her daughter, who didn't have a choice and who, in some ways, dies because of Mr. Harvey's jealousy and desire to destroy her and the perfect world he can never share.