There must be some non-supernatural secrets too... It's difficult to think of though...
1) A rich merchant wanted to enter the aristocracy, so married a noble's daughter. However, all he could get was an idiotic wife, as the noble gave him (in fact sold him) an almost moron daughter. Nonetheless, through this marriage the merchant is trying to get a nobility title, spending/wasting huge sums of money to influence appropriate people in the highest circles of society. He seems that he could be soon succeeding in this endeavor and become a baronet. However, here is the problem (and thus secret): marrying a noble woman was the first requirement, but he somehow was tricked in marrying this one. He made up his mind about it, but always had to be careful that nobody knew he is married to a stupid and vulgar woman (despite she is of noble birth). So the wife remains hidden in the castle, and sometimes the merchant is obliged to employ an actress to act as if she was his wife (telling the actress some lies to justify it). Would this be discovered, he would be seen so ridiculous, that he would be mocked by all the nobility, and would subsequently lose reputation and thus his business would suffer greatly.
2) Some noble that everybody believes to be rich is not. He doesn't have a gold mine in some distant new-world. Everything about him is fake and false, using forgery and bribery to steal from the state and from whomever would deal with him, etc. Among other swindles, he has drawn a lot of money from the above merchant, and intends to give him a false nobility title. It will look real at first sight, but would reveal a forgery if careful investigation was ever made.
3) Miss Victoria is not a damsel despite successfully pretending the contrary. This is Victor, an homosexual from the gentry, masquerading as a young and pure virgin. He has kept the secret for a long time, and because of his mother who wanted a girl and lied to her husband who incidentally was often away for work, Victor's father himself doesn't know about it!! The problem however, is that Victor's father thinks it's now time to marry his "daughter"...
4) John Sheltrun Junior is an impostor too. As a destitute orphan, he semi-accidentally killed a noble child when he was but 6. The slain child had no longer his parents too, but an old tutor who didn't want to lose his well paid job. So, tutor took the destitute child and raised him as if he was the real John Sheltrun Junior. This required much work to make it so nobody would notice. At first John Sheltrun Junior was said to be extremely ill for longer than a year, after which tutor brought him to a better place far from town, etc. Eventually tutor died and false John (who conveniently forgot his real name) reached adulthood. Now John Sheltrun Junior is in town to reclaim the heritage of the child he killed many years ago, and there is nothing that should prevent it.
