In the days before the D-Day landings, there were several recorded cases of local kids (in the UK) being found to know dangerously intimate details about the upcoming operation.  Generally, they learned these from listening in on off-duty servicemen talking to each other.  Some of the kids were found to have kept detailed records.  Thankfully, Axis intelligence services were about as competent as the rest of their government.  But it goes to show how difficult it is to keep something a secret when lots of people know it.  Especially in cultures where people feel like they can speak their minds without judgement (freedom of speech and freedom of opinion).  Even if they know they shouldn't be talking about military secrets, if they've been raised to talk honestly and openly, that's a hard habit to break.  With modern social media, it's even easier.  Just watch the people who come into contact with important secrets.  This is part of what's called "Open-Source Intelligence" or OSINT.  Even the smallest, weakest nation of the world can do it effectively.