Thanks, Mr. C. It is in the album art column, they are always (as far as I can see) a member of a stack, and when the file is clicked on to play, a "something went wrong...file not found" message appears.
Right now I manually click on another tab, select audio, search for the file, drag it to the playlist and then delete the "red x'd" version. This is, obviously, a slow, inefficient way to fix this. I'm trying to learn how to do this in a more efficient way, if there is one.
Any suggestions?
Thanks again for your help.