It was a while back, so my memory is a little fuzzy, but I'm sure I ran into a problem whereby, after moving a shedload of album art to a new home, thus breaking MC's database links, instead of MC proclaiming "no coverart", it displayed the file address of the missing image instead. Doing a "quick find in coverart directory" didn't work on files where MC believed it had art when in fact, it didn't. I had to search on [image file]=<insert path here common to the old address, but not the new one>
In my case it was easy because I'd changed drives, so the root entry was all i needed. [image file]=c:\
Once you have that list, select them all, right click and remove coverart. Even though I knew they wern't there, I chose not to delete from the harddrive, it seemed a redundant question to ask in this case, so, being forced to answer, I erred on the side of caution!!
Now, the quickfind option should work.
I may have that totally wrong mind you, best to test on one file and see what happens first.
edit:
It just occurred to me this might have been a problem caused by the fact that when MC saves coverart, it names the files as [artist] - [album].jpg, but if you subsequently edit the album name, MC maintains the coverart link, but does not update its filename, which in this scenario, means that the quickfind option won't work for those albums, because the files won't have the correct name. quickfind should work for you, the [image file] search help to identify any rogue albums.