Orphaned thumbnail(s) may be shown after you rename the filenames outside ACDSee, e.g. in Windows explorer, and then filter the images with the options such as uncategorized, unlabelled and unrated.

To address this issue, run the database optimization to clear the orphan files with the following steps:

Click Tools | Database | Optimize Database.
Click Next.
Turn on both checkboxes and click Next to start the database optimization