Opus not showing all files (due to hidden files)

You must have changed settings in both Opus and Explorer to have it that way around:

  • Explorer doesn't show hidden files by default.
  • Opus does show hidden files by default (unless they're also flagged as system).

You can change it under Preferences / Folders / Global Filters (or via the Folder menu). More detail in this FAQ: Toggle display of Hidden / System files

As for how the files had the Hidden attribute set on them, the simple answer is that something set it on them. I couldn't tell you what set it on them. It might be the program that wrote the files to the CD. Or something might have set it on them before they were written to the CD, and the CD-writing software carried it over. Impossible to guess. You'd have to ask the person who created the files or the CD.