In talking about similar themes and characters in the Christie canon, one incident stands out from the rest. In no less than three novels, The Pale Horse, By the Pricking of My Thumb, and Sleeping Murder, she has an elderly woman in a nursing home asking about the child behind the fireplace.
While it may seem as if the books were similar due to the timing, it must be remembered that Sleeping Murder predates its release by some 30 years. Therefore, Christie was writing about this particular incident over the course of some three decades.
The incongruence of the incident occurs in The Pale Horse and Sleeping Murder, where the incident does not have anything to do with the conversations at hand. The incident occurs while the protagonists in the book wait for someone else. The incident is thrown into the action in these later books as a jarring note. This marks a change from Christie’s earlier works where she makes every word count – every word driving home to the denouement.
It’s only in By the Pricking of My Thumbs that Christie investigates what that crime entails. Tommy and Tuppence clear up the mystery of the child. In reviewing John Curran’s Agatha Christie: Murder in the Making, he shows that Christie took notes on the “your poor child” behind the fireplace, but he too cannot come up with a satisfactory explanation for its appearance in three novels.