The Mystery of the Hidden Painting — Gertrude Chandler Warner

One warm night four children stood in front of a bakery. No one knew them. No one knew where they had come from.

Henry, Jessie, Violet, and Benny used to live alone in a boxcar. Now they have a home with their grandfather, and they find a mysterious painting hidden in the attic.

The children learn that it is a portrait of their grandmother, whom they never knew, wearing the necklace Grandfather gave her when they were married. The necklace disappeared on their first anniversary -- and was never found. Can the Boxcar Children track down a necklace missing for so many years?