I'm not sure if Las Vegas has done mass testing already, if not the scenario you just laid out is possible. I think they should have done mass testing before reopening the casinos.
That is my question too because if they don't do that, I think the possibility to have a hidden carrier will wide open since many people will come to that place at once. Even if they do a fast checker before they go inside that place, that doesn't guarantee for infected people can be found or not because the carrier doesn't have any sign if they are infected or not.
It could be really implemented by casino themselves. And everyone should be responsible here. We don't want to hear an outbreak, the federal government really did a big risk here by opening up as early as June without any vaccine for the public.
It is a serious matter in that city because it's related to many people. Before they open the casino, they really need to test every people, no matter if they already have been checked by the local hospital or not because the virus can attack any people, whether they wear masked or not. I hope the local government can handle this before anything bad happens.