I double-checked the log entry, and found I had looked in the wrong location. Was able to find the sparse-image file, and delete it. All is well now, with 14 GB of freed up space.
Just for the record, though, Super Duper did present me with the option of creating the sparse image to my boot drive as my first option -- which is probably why I chose it. I don't know how to replicate the procedure, which occured right after registering the copy, nor do I think I want to ;>)...
Thanks for your help, David.
