A British man is ridiculously attempting to sue Apple following a divorce, caused by his wife finding messages to a prostitute he deleted from his iPhone that were still accessible on an iMac.
In the last years of his marriage, a man referred to as "Richard" started to use the services of prostitutes, without his wife's knowledge. To try and keep the communications secret, he used iMessages on his iPhone, but then deleted the messages.
Despite being careful on his iPhone to cover his tracks, he didn't count on Apple's ecosystem automatically synchronizing his messaging history with the family iMac. Apparently, he wasn't careful enough to use Family Sharing for iCloud, or discrete user accounts on the Mac.
The Times reports the wife saw the message when she opened iMessage on the iMac. She also saw years of messages to prostitutes, revealing a long period of infidelity by her husband.
If you work for DARPA and send anything near sensitive materials over iMessage, and something happens over it, that's on you for being dumb.
People who work with sensitive information should know better than to use personal communication methods. If they don't that's their fault.
Any workplace where this is a worry, this should and probably will be drilled into your head from day one.
The point is to divorce the situation from the cheating aspect, so that people can be less emotionally invested in the outcome. Plenty of jobs that handle industry sensitive information do so over normal communication lines. DARPA was possibly a poor example because the assumption from you is that anything handled by them requires a clearance (which I wouldn't consider to be true). Something as simple as tracking the whereabouts of a naval ship can and has been done via Facebook posts from people onboard or their families.
The point is that it wasn't clear to the user that their information wasn't being deleted in real time and that's poor transparency on the part of the company because a lot of users probably assume the same just based on the comments I see here.