I think that checking files, even though they are game files, or whatever files they are, it's illegal. If you know 100% sure that they are checking your files, you can sue Nexon.
But I think that they are going to take some undercover GM's in-game rooms, and they will take video-footage and/or maybe something like this:
Report --> Message to GM --> They enter that room without information, free movement etc. so they can check every place --> video footage --> ban