When Guest mode is turned on, Chromecast emits a special Wi-Fi beacon. When a Chromecast-enabled app is launched on your guest's mobile device, that device detects the presence of the special Wi-Fi beacon and shows the Cast icon 
Your Chromecast then generates a random 4-digit PIN that is required to cast to it using Guest mode. When a device nearby tries to connect, the Chromecast automatically transfers that PIN using short, inaudible audio tones. If the audio tone pairing fails, your guest will be given the option to connect manually by entering the 4-digit PIN found in the Google Home app.
Learn more about the guest mode PIN.