You are seeing that because you are on a shared wifi and someone else on that wifi is casting.
That's just like if you were home and someone else in your house started casting; you would see a notification then too.
But, that also tells me that your hotel has pretty lax security settings on their WiFi; they are allowing all their guests' devices to see each other. That doesn't mean you have been or will be hacked... but it does mean you aren't protected very well while on their network.
