mystery guest

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.

On this show the celebrity guest came out dressed in a costume that hid his or her identity. Based on the costume the panel asked questions until they figured out the identity of the guest. The mystery guest in this sentence is dressed as Cap'n Andy, the captain in Jerome Kern's classic Show Boat, and the set was the deck of a paddle-wheel steamer.