The operative word is willfully retaining them. If they were willfully retaining them, then they might choose to charge them. But when they didn't know they were there, and the second they discovered them returned them, there is no crime. End of story.
From your article you linked to:
(a)
Whoever, being an officer, employee, contractor, or consultant of the United States, and, by virtue of his office, employment, position, or contract, becomes possessed of documents or materials containing classified information of the United States, knowingly removes such documents or materials without authority and with the intent to retain such documents or materials at an unauthorized location shall be fined under this title or imprisoned for not more than five years, or both.
This is so abundantly clear, it is ridiculous you are making an argument about simply having them, regardless of if they were intentional or not, when the statute is clear about intention, then you are just desperately wanting to believe something the law itself does not state.
