I had an opponent go from full strength connectivity to disconnected three times just before they made critical moves. I am almost certain they were consulting an engine, probably whilst playing on their iPhone where they had to momentarily close the app. I know there's little that can be done but it's irritating nonetheless.
https://support.chess.com/article/648-what-do-i-need-to-know-about-fair-play-on-chess-com
If you feel an opponent is using disallowed resources, you should report them.
https://support.chess.com/article/209-how-do-i-report-someone
However, discussions about cheating, potential cheating, or cheat detection are not allowed in the general forums.
I had an opponent go from full strength connectivity to disconnected three times just before they made critical moves. I am almost certain they were consulting an engine, probably whilst playing on their iPhone where they had to momentarily close the app. I know there's little that can be done but it's irritating nonetheless.