Question

What is Mrs.Frankweiler last secret?

Answer 1

Mrs. Frankweiler had given away a virtually priceless Michelangelo to the Met on purpose.

Answer 2

Mrs. Frankweiler has one particularly big, honkin' secret: whether or not the angel statue was carved by Michelangelo. Even though the museum officials are all clambering to find out the truth, she refuses to hand over the documents.