" We have detected that one or more of your recent opponents has violated our Fair Play Policy.. "
We've recently noticed......................

when I get a points refund I look back through my game history and see who's been banned. Usually tallys up.

They have an AI system they use to recognize cheating, but I think humans also get involved too at some point, I'm not sure of all the details though.

https://www.chess.com/article/view/online-chess-cheating
https://support.chess.com/en/articles/8568369-what-do-i-need-to-know-about-fair-play-on-chess-com
That said, discussions of cheating, potential cheating, or cheat detection are not allowed in the general forums.
If you would like to discuss join the following club.
https://www.chess.com/club/cheating-forum
I occasionally get this message but how does chess.com know someone has been cheating?
Do they tell the cheater they've been caught?
What punishment do they get?
Why don't they tell me who it was and what game it was so I can look back at it?