I’ve gone on at a little length there because I want to make it clear: this classification is pretty heavy stuff, with the potential to cause “exceptionally grave damage,” and containing information that can be identified as coming from both Earth-based and satellite-based collection, that shouldn’t be shared with any foreign national.
This information would always be air-gapped. There is no (legitimate) way that a computer system could be connected to TS//SI//TK//NOFORN data and to the outside world.
What can happen is that someone copies information, onto a piece of paper or a thumb drive (actually systems that can handle TS shouldn’t have thumb drives either, but it’s too easy to sneak one in or out) and then copied into an email in an uncontrolled system – a cell phone or a laptop or an iPad. The person doing it has to know that it’s coming from a secure system, has to know how sensitive the data really is; they go through lots of training, repeated reminders, and come and go to the office through a freaking vault door that would do credit to a bank.
It has to be done on purpose, and it has to be done knowingly. There has to have been conscious intent to do it.