You haven't really asked a question, so I'm going to generally describe what your rights might be if you were terminated. You may have rights as a whistleblower. In California, you are protected from retaliation for reporting activity you believe to be illegal even if you only report it internally (as opposed to reporting it to governmental authorities). However, you must have a good faith belief that the conduct you are reporting is illegal. It is not enough that the conduct simply violated an internal company policy. Even if you do not qualify as a whistleblower per se, you may still be able to state a claim for wrongful termination in violation of public policy if you can establish that the company's handling of customer documents is a fraud on those customers.
Since there really is not enough in your post to analyze your situation properly, I would suggest that you contact an employment lawyer to discuss your situation. Many of us offer a free initial consultation, and it sounds like you could use one.
You need to re-post to allow us to understand what you want to know. There is nothing wrong with an employer requiring an employee to retain as confidential what goes on in the workplace, with some important exceptions.
Good luck to you.
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An employer is free to restrict activity of an employee provided they are not requiring you to engage in illegal activity or violate public policy. Mishandling client information may not be illegal, so telling a client about the behavior may not give you whistle blower protection. As suggested you should consult an employment attorney who practices in your area. There are a number of us employment attorneys in the Sacramento area, go to the find an attorney tab for some help.
Best of luck.
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