Google recently released an update to the Android Chromecast app (now called Google Home) that introduces a new annoyance. Whenever someone on your network broadcasts a Chromecast, each the android device is notified of this.
You’ll have to disable this notification on a per-device basis, but it’s not hard to do. If there is a notification on the screen, swipe down from the top of the screen to open the notification drawer. Drag down the Chromecast recording with two fingers and tap the gear on the left side. The only option here is Show remote control notifications which you can disable.