That's the name of a book of short stories by Alice Munro but it's also an underused expression. I was just thinking lately about how many open secrets there are. The damn things are everywhere!
When I say open secret what I mean is something you trick yourself into believing is true, even though you know it isn't actually true. For example:
1) Most of the time, when people tell you things, they are being honest.
2) Everyone actually has your situation and not theirs in mind when they give you advice.
3) Money can't buy happiness.
4) Your life may improve very suddenly all by itself. Any day now.
5) Living in a state of perpetual loneliness is okay. It's just your attitude that needs to change.
6) Love is something that defies logic and you have no control over whom you go for or whether or not they are nice to you.
7) Everyone recycles, especially people who have loads of different-coloured bins in their house.