state trade secret laws. As for the duty to "discover" trade secrets ... an employer usually makes it clear what is a trade secret. If not, the categorization (of what is a secret) may become an issue if an employer decides to sue.
Why do employers use NDAs? Employers use NDAs -- in addition to state laws that protect them -- because it's possible to obtain extra benefits when suing, including increased damages, payment of attorney fees and a guarantee as to where or how the dispute will be resolved.
Not to put to fine a point on it ... The Dear Rich Staff answers many trade secret questions (over 20 so far). Use the search box on the right to find them. We also have a thorough trade secret site with lots of free NDAs!