I don't think that casinos use hidden mining tools, at least not those respectful ones. Besides that could be detected if check your computer thoroughly. Although I'm not fan of auto betting but for different reasons I don't think this feature is misused.
Casinos are working well with me and my computer didn’t heat that much but I don’t think its possible for them to use our own GPU to mine, I don’t think technologies are too advance like this. I don’t gamble on autobet because you can’t have fun on that and technically you are gambling for the sole purpose of money and not to enjoy playing, I still choose to bet on my own and enjoy all the losses.