Which idiomatic phrase can be used to express 'showing all the hidden stuff' (it's supposed that nobody should find that out, some scandal things)?
Uncover the skeletons in the closet or, similarly, show where the bodies are buried.
From The Telegraph: "A married magistrate was discovered defrauding a charity she worked for after her lover of 10 years blew the whistle." More common is the noun form: whistleblower.
Do you want to describe an outsider discovering secrets or an insider revealing them? Most idioms that I can think of lean one way or the other.
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1+1 for raising that crucial distinction between discovering and revealing, which I too think applies to most idiomatic forms. Unearth, ferret/worm out, etc. as opposed to blab, squeal, sing, rat, snitch, split, etc., etc.. As you can probably guess, I also think there are more (mostly negative) ones for the latter! – FumbleFingers Oct 16 '11 at 15:21
If you're letting someone in on the secret goings-on, you can be said to "open the kimono."