After returning an ebook to a library using ADE 2.0 on a Windows Vista PC, I checked my library account and saw that I had no ebooks checked out (so the ebook was successfully returned). I attempted to delete the ebook from ADE and it warned me that the book was unreturned. I went ahead and removed it anyway.
I also had to manually delete the associated .acsm file and the associated document file on the Kobo ereader.
Why did this returned ebook have an unreturned warning message? I think that it is a bug in ADE 2.0.