You just learned that an employee secretly formed and operated a competing business while employed by you. Is there a claim against the competing business or just the employee? Most likely there are viable claims against both. The fiduciary duties of the employee are likely to be imputed to the company he or she formed.
Breach of Fiduciary Duties
Similar facts were before the court recently in an unfair competition and breach of fiduciary duty case, Vibra-Tech Engineers, Inc. v. Kavalek, Civil Action No.: 08-cv-2646, in the United District Court for the District of New Jersey. (opinion here) A vice president and director of Vibra-Tech, along with his wife, formed two businesses. One of the businesses sold equipment to Vibra-Tech; the other competed directly for the same customers. Vibra-Tech, of course, had no idea that one of their executives was involved in the two businesses.