If I were you I would seek legal advise before doing anything. While the detection and investigation of fraud is a very important area of a company especially given the nature of  bogus employee's being on payroll, the rights and legal avenues that you pursue in this matter are just as important for you so as not to limit your ability to recover the loss but also increase your liablity by perhaps violating another employee's rights. To ensure that your rights as to recovery and recourse in this matter are maintained and not futher your problems with another illegal act or create paranoia withing your company with questionable tactics. Payroll fraud is not only a very important issue it also has tax ramifications that may open another can of worms. To ensure that your rights to recovery and prosecution are maintained without furthering your problems with potential additional illegal acts performed by your actions with potential unacceptable and perhaps even illegal actions and performance by you in this investigation. You most certainly do not want to create a greater or additional problems by violating other employees of your company rights and even perhaps create more legal and financial costs that you perhaps could become subject to concerning this matter. An attorney in your area would be able to direct you in the proper legal directions to assure limiting further problems. A good CPA would also assist in the determination and value of the exact loss for your recovery and restitution.

 You need to either report this to someone higher or the police this is illegal or get some advice from a solicitor the first hour is free you will walk out armed with knowledge in where to go from here