Noun for someone whose secret has been discovered?

Wouldn't they just be a liar? If they are not found out they are not a liar (to you).

Debunked might fit.

It is usually used as a verb or an adjective. It can be used as an adjectival noun also but that usage is not common. (For example: debunking the debunked.)

Debunker is the one who debunks.

In a very few sources, debunkee is used as a neologism or a nonce word for the one who gets debunked.

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debunk

: to expose the sham or falseness of

: to show that something (such as a belief or theory) is not true : to show the falseness of (a story, idea, statement, etc.)

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Disclosed comes to mind also but it is used as an adjective mainly.

made known (especially something secret or concealed); "the disclosed purpose of their wicked plan"

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Note: Of course they can be called many things like crook, sham, phoney, double-dealer, counterfeiter, faker, scammer, fraudulent, trickster etc. depending on the context but the key point is being found out/discovered.

Hmm, I can't think of one word that specifically means that. If you mean someone who is known to lie in general, the common phrase is "known liar". If you mean that he has been caught lying about one particular thing, I don't know any word or short phrase for that. You'd pretty much have to say, "Senator Jones was caught lying about his campaign contributors" or whatever the story is.

Fraud comes to mind.

To our dismay we discovered Jacob was a fraud.

I say fraud because it almost always is used in the context of someone who has been exposed.

If that person was under oath when he/she lied and was debunked later on, the usual, legal term is perjurer.

perjury: the crime of telling a lie in a court of law after promising to tell the truth.

Alternately, consider impostor for someone who has been lying about or keeping his or her real name, identity, etc., a secret until he or she was debunked, and discoveree for someone who has been found out as lying, cheating, or keeping a secret.

impostor: a person who practices deception under an assumed character, identity, or name.

discoveree: (rare) one who is discovered.

While the finding out of the lie isn't a requirement, the word charlatan is probably appropriate.

Why not simply apply a couple of the world's most famous proper names to this situation?

"Nixon" or "Clinton" both scream to me as exceptional options, as in:

"He certainly has proven to be a Nixon."

"After that Clinton I doubt I'll ever believe him again."

Consider:

Racketeer

Liar

Hypocrite

I don't think there is word for what you've described.

Someone who has been lying is a liar, but that applies whether the lie has been discovered or not. The same applies to fraud, charlatan, or perjurer. I agree that these words are most often applied to people whose lies have been discovered, but that's because we don't call people liars unless we know they've lied. (For the same reason I don't refer to a box-of-kittens as a box-of-kittens until I've opened the box and seen the kittens inside, though it has been a box-of-kittens all along.)

But the original question is not referring to only liars. It refers also to someone who has kept a secret. Keeping a secret isn't the same as lying, and someone who has kept a secret isn't necessarily a liar or a fraud. (In the sense that my doctor is not a liar because he refuses to discuss my medical condition with you.)

Culprit: a person who is responsible for a crime or other misdeed. "the car's front nearside door had been smashed in but the culprits had fled"

...the cause of a problem or defect: "low-level ozone pollution is the real culprit"

Google: Define culprit