This is very useful. Matt Goddard posted this on Facebook, and I though I’d pass it along. Create a Library level Smart Collection, and set it to search for text that includes “missing”. It will find and list all missing media. Also, T. Payton points out that you can perform this search in the Timeline Index as well. Thanks guys! 🙂
Final Cut Pro X Babbling 
Smart collection?? Simply enter it into the search field. Done.
Yep. Having it saved is useful for ongoing jobs though. As i said, works in the TL Index as well as a search term.