mystery guest

noun2

1. •
   A person hired by a company to pose as a customer in order to evaluate the quality of its service.

   • For example, an amusement park may hire an outside marketing firm to send a mystery guest to the park to evaluate his or her experiences with the park's sales and service employees. — 2007
2. •
   A guest star whose identity is kept secret.

   • Guess who will be our mystery guest this week.