ICL VME Diagnostician here, 1980's
We had a constant complaint from an unnamed government agency that we couldn't resolve a repeated system crash.
They would send redacted system dumps on paper, every hex combination on the dump which looked like it was an alphabetic of numeric character was neatly cut out using a scalpel. They had actually specifically trained a couple of people to match the characters but not understand them (in theory) needless to say we could follow the trail through the dump so far until it reached a hole.
We did have a couple of people with very high security clearances but apparently this stuff was 'eyes only'