I’m guessing that the render files of that FCP made of your original media were still stored, so you could still see the material. If you delete, move, rename or even *edit* a file outside FCP, then it will also say Media Offline
What gets me about FCP (I use Pro 5) is that when I edit a bit of media (eg I normalise a WAV in a project using an external editor), I get the “media offline” message – fair enough. But I have to manually show FCP where it is, or let it search the whole disk. It *knows* where the original file is, so I wish it would just say “this file has changed – update Y/N”. Applications like Cubase have done this for years.