If you believe that you have been subject to an unreasonable invasion of your privacy at work, your most powerful weapon may be to file a lawsuit against your employer. The U.S. Constitution (if you are a public employee) and your state's constitution and statutes may provide you with some privacy protections at work, but often it will be up to the courts to determine whether an employer's actions are legal in a given context.
When judges evaluate whether a particular workplace search or surveillance action is legal, they must balance two competing concerns: the employer's justification for taking the action and the worker's reasonable expectations of privacy.
As such, your best argument will be that you legitimately expected -- based on your employer's policies and past practices, and common sense -- that the employer would not search certain areas or take certain actions. For example, you may have a high expectation of privacy in the employee restroom or a changing area, particularly if your employer has not warned workers that these areas might be monitored.
Your argument will be stronger if you can show that, in the process of collecting information on you, your employer engaged in one or more of the following practices.
Deception. For example, your employer asked you to submit to a routine medical examination, but mentioned nothing about a drug test. However, the urine sample you gave to the examining physician was analyzed for drug traces, and, because drugs were found in your urine, you were fired.
Violation of confidentiality. For example, your employer asked you to fill out a health questionnaire and assured you that the information would be held in confidence for the company's use only. You later discover that the employer divulged the health information to another prospective employer who inquired about you.
Secret, intrusive monitoring. For example, your employer installed visible video cameras above a supermarket's cash registers. This is usually considered a legitimate method of ensuring that employees are not stealing from the company. However, installing hidden video cameras above the stalls in an employee restroom would probably qualify as an invasion of privacy in all but the highest security jobs.
Intrusion on your private life. For example, your employer hired a private detective to monitor where you went in the evenings after work. When the detective reported that you were active in a particular political organization, the company told you to resign from the organization or risk losing your job.
To find out about privacy protections in your state -- and what to do if you believe your privacy has been violated unlawfully at work -- contact your state department of labor.
For an invaluable reference that covers all of your employee rights, get Your Rights in the Workplace, by Barbara Repa (Nolo).