Here's an interesting pickle. When shipping classifed documents (DOD, etc) using the US Postal Service is the only acceptable provider, due to the secure facilities (and other aspects we won't go into). I wonder how that's working out these days. No one, other than the sender and recipent, knows the material is classified. No one else along the chain has any idea of the contents of such a package.
I know when receiving packages thru UPS and FedEx, there've been times when it was obvious someone opened the package, even more obvious that something was removed. But no way to prove it. But when something goes wrong with a letter or package with USPS, I always received a letter/notice with explanation and some level of remediation.
Reason I bring this up now, last spring a number of the common-mail boxes (such as apartments, condos, neighborhoods) were broken into and the mail stolen. Unfortunately the notice went to the management company and it was more than 2 weeks before the mgt company informed the residents. Had to pick up mail at the post office for 3 months, because that's how long it took for the management company to get get estimates and install a new door that satisfies the USPS. Second unfortunately, we're still stuck with the same management company, but that's for another discussion.