Upvote:12
According to Section F of the instructions on form DS-82 for a US passport renewal: (emphasis in original)
You may receive your newly issued U.S. passport book and/or card and your citizenship evidence in two separate mailings. If you are applying for both a book and card, you may receive three separate mailings: one with your returned evidence, one with your newly issued book, and one with your newly issued card. All documentary evidence that is not damaged, altered, or forged will be returned to you. Photocopies will not be returned.
So it is completely normal and expected to receive up to three mailings if you are applying for a passport book and card: one with the book, one with the card, and one with your old passport and other submitted evidence. They may arrive on different days, and I would not worry if it has been only a couple of days since you received the first of these mailings.