UPDATE: whilst moving/copying files around to start with a new set, I discovered the file I had renamed as damaged would open when moved out of it's original folder—the folder name: "coyName last FY". Deleting the folder and creating a new one by the same name didn't resolve the problem.
Creating a new folder named "coyName prev FY" and placing the file into that folder enabled the file to open.
Is it possible that because the folder has a function name, FileMaker is a more than a little confused?
Maybe I should have wasted waited another day before posting. Now FileMaker won't quit because the file that is being closed is in fact hidden as if their is a parent file calling on it via a relationship. There are definitely no other files open.