I'm posting this here in hopes it might be helpful to someone at some point.
I've had a long standing problem with my Lightroom catalog where it displayed a folder with a "?" on it. Despite all attempts to delete the folder, remove it from Lightroom, optimize the catalog, etc. etc. I couldn't remove it. It didn't seem to cause any issues, but I've never been comfortable with having a level of corruption in my primary Lightroom catalog. With some more internet searching, I found a solution this weekend.
1) In the Library module, I selected all the folders. I then deselected the corrupted ones. 2) I went under File: Export as Catalog 3) I unchecked all the boxes 4) It creates a new catalog, with the corrupted folders gone!