Can't delete asset when it's not in use

Having educated myself about the benefits of importing MPEG-2 files into Encore so it doesn't have to transcode for DVD, I'm having a small problem.

One of my projects had an opening menu animation that was a large QuickTime file. I've since imported an MPEG-2 as a replacement and linked the motion property of the menu tab to the new file.

However, whenever I now try to delete the old .mov file from the asset panel, I get an error telling me that the file can't be deleted because it is still in use by the project. But where?

(I'm using CS5.5 on a Mac, by the way.)