Daniel,
Thought I would revisit this one. On page 235 of the FCP X Help Manual it says the following ...
Manage media files ! Solutions to common media management issuesAlert icons
When you see red frames and a yellow alert triangle in a Timeline or Event, Final Cut Pro is alerting you that part of your project or Event is missing. There are many reasons that clips and Events, media files, and effects can be missing: moving projects and Events between computers and actively managing your Final Cut Pro Events or Projects folder usingthe Finder are two common reasons.
For more information about where Events, clips, projects, and files are located, see Where your media and project files are located.
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On down that page, it then goes on to say the normal scenarios that have been discussed on this discussion. However there was one that was somewhat new to me ... that is that another program other than FCP X could have modified a media file in an event and caused this sort of icon. Supposedly that means the clip would still be represented in the event, but that FCP X would think there might be a problem there. This is all the news I have ... just thought I would throw it out there. Like you, it bugs me when I cannot explain why something is there 🙂 Best wishes.
stephen