Many (approaching 50) years ago, wandering around LHR T1 waiting for the last flight of the day to EDI or GLA, I would often find an unattended stack of A4 sheets listing the assignments of aircraft to flights for the next day. This appeared to be intended for internal use, but not a closely guarded commercial secret. This was well before the days of on-line everything, so today the equivalent information is available only to BA staff and skilled hackers.
This was so long ago that it was before the introduction of lounges, and even before pre-boarding security checks. The assumptions appeared to be that nobody would expect a BEA flight to be able to take them to Cuba. On the other hand, there was a coal mine called the Havanna Drift (with two"n"s) just off the eastern end of the runway at NCL. Its workings extended under the airport, and during its active life from about 1950 to 1970 it reduced the elevation of the eastern end of the runway by about six feet.