By the Pricking of My Thumbs — Agatha Christie

There was nothing remarkable about Mrs. Lancaster's departure from the Sunny Ridge establishment for elderly ladies. Except for the painting she left behind--and a single, disturbing question that echoed in Tuppence Beresford's mind...

Tommy and Tuppence Beresford never looked for danger, but it always found them nonetheless. How could they know that a dreadful secret from the past was all too murderously alive?