Phantom Files, also known as Hidden Game Files, are files that are purposely hidden or improperly titled by Rockstar Games intended to disguise and hide myths from those viewing on their PC.
There are thousands upon thousands of files per GTA game, making it difficult for a single person to open and view every one. Most people just skim over and glance at the file names, looking for one to stand out. In most cases the person comes up empty, as they didn't find a file titled with the name of the myth they are trying to find.
Because of this, it has become widely believed in the myth hunting community that Rockstar anticipates that people are going to search the game files, so in an effort to keep a myth secret, they disguise the name as something that it is not and possibly place the file in a category where it wouldn't belong in (like a model being placed in the audio section.) Others argue that Rockstar can hide files from view entirely, making the file impossible to find.