Actually, a individual who is not in a federal job and who doesn't have a U.S. clearance is under no obligation to return or otherwise protect any classified information of which they may accidentally possess. In the professional world, a reporter, despite the best efforts of some judges, prosecutors, and administration officials, is under no obligation to return or otherwise protect any classified information which an "insider" may leak to them. For the leaker, it's a different story.

In the amateur world, anyone who winds up with a package containing classified material accidentally delivered to their home by the USPS is under no obligation to return it. I remember one occurance of this back in the mid-1990s. One defense contractor was (lawfully) shipping an envelope containing a classified document from one plant location to another. The post office accidentally delivered it to a private residence. The wife got home first and figure the package was for her husband. She opened the outer wrapper and was immediately confronted with the classification markings on the inner wrapper along with the standard "Do not open and call this number if this isn't yours" label contained on inner wrappers. She stopped right there. When her husband got home, he saw the package and called the number on the label. Some Air Force guys came to their house, thanked them for doing the right thing, and retrieved the package. End of story.