In some respects, I think a lot of secrecy--don't know what %--is delusional arrogance.
Most individuals think that those around them do not know their ugly parts much at all. Sometimes they don't. But in my experience, most of the time, co-workers, relatives, closer friends--at least the discerning, intuitive types--know, sense or at least have a good guess.
Most of us are a lot more open books than we could ever imagine.
I wonder if a similar phenomenon is somewhat true in terms of state secrets. Collect enough public data and a lot of decent inferences can be made.