By the Pricking of My Thumbs by Agatha Christie

While visiting Tommy's Aunt Ada at Sunny Ridge Nursing Home, Tuppence encounters some odd residents including Mrs. Lancaster who mystifies her with talk about "your poor child" and "something behind the fireplace".

When Aunt Ada dies a few weeks later, she leaves Tommy and Tuppence a painting featuring a house, which Tuppence is sure she has seen before. This realization leads her on a dangerous adventure involving a missing tombstone, diamond smuggling and a horrible discovery of what Mrs. Lancaster was talking about.