The Case of the Counterfeit Painting, Paperback/Steve Brezenoff

When Clementine Wim spots a famous painting being carried away from the Capitol City Art Museum, she knows something is wrong. But when she arrives at the museum, the painting is hanging right where it should be. No one believes what Clementine saw: not even her mother, an assistant curator at the museum, or her friends. It's up to Clementine to convince the others and determine fact from forgery before it's too late.