The emailer is trying to find out what it is and what it means. I don't recall seeing this before in the Timeline. It looks like a formal Approximation symbol ≈ (option+x) to the left of the thumbnail and has actually moved the thumbnail to the right. Has anyone seen this before or know the cause?
UPDATE: Solved! It's a "mixed-sync indicator" and explained (with screenshots) in the FCP manual:
Moving a single pair of items out of sync results in a single out-of-sync duration, with out-of-sync indicators with positive and negative durations in both the video and audio items.
If you then move a second pair of audio items out of sync by a different amount, each audio item that is out of sync from the anchor item has an out-of-sync indicator noting its individual offset from the anchor item—in this example, the video item. The anchor item displays a mixed-sync indicator with no duration. This tells you that multiple linked items are out of sync by varying amounts.
Thanks to everyone who offered help in the comments!