A secret society is a club or organization whose activities, events, and inner functioning are concealed from non-members. The society may or may not attempt to conceal its existence.
Yes, of course the lower echelons of Freemasonry are completely oblivious and makes for plausible denial for the organization as a 'hole' (pun-intended).
Wow - I never expected to find Freemasons on HN. Don't know why but i thought it was a) dying out and b)it was for old people. Well older than my 43 years of age :)
Disclaimer: I'm a Freemason.
Disclaimer 2: No, I don't know where any treasure is buried. It's really not that sexy.