Under the Egg by Laura Marx Fitzgerald

Theodora Tenpenny lives with her mentally unstable mother and her beloved grandfather. They have very little money, and when Theo's grandfather dies, she wonders how she will take care of her mother by herself, with no source of income. With his dying breath, however, her grandfather told her about a treasure “under the egg.” Hopeful, Theo looks underneath all the eggs—real or otherwise—in the house. Only when she accidentally spills rubbing alcohol on her grandpa's painting of an egg does she realize what the treasure is—and that it might have been stolen from the museum where her grandfather worked. Was her beloved grandpa a thief?  (Booklist)