Google has added a new feature to its Chromecast dongle: the ability to beam content from guests even if they aren't connected to your Wi-Fi network. Dubbed guest mode, the feature is currently limited to Android 4.3 or newer devices, and it works like so:
Okay, so that's not terribly informative. Here's a more detailed description from the Chromecast help pages:
When guest mode is turned on, Chromecast emits a special WiFi beacon. When a Google Cast Ready app is launched on your guest's mobile device, that device detects the presence of the special WiFi beacon and shows the Cast icon in the application. Upon tapping the Cast icon, casting to a 'Nearby Device' will be listed as an available option.
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 on your Chromecast backdrop and in the Chromecast app.
Unless you have extremely thin walls, the "inaudible audio tones" should help keep your TV off-limits to neighbors and the like. Always requiring manual PIN code entry would be even more secure, but if someone's already in your living room, making them jump through hoops seems a little pointless. Besides, guest mode looks easy enough to disable when it's not needed.