Drupal: core module forum not listed

Recently, I had the problem that the core module forum was not listed in the list of available modules under /admin/build/modules. It was simply missing. It had me puzzled initially because the module was actually present in the default location modules.

After looking around I noticed an out of place file forum.module directly in the modules folder. I had some FTP issues that night and I must have dropped it there mistakenly. After deleting the rogue file the forum module showed up again in Drupal's list of optional core modules.