Thanks, that's a helpful explanation (although I had to reread it a couple of times to properly understand it!).
It sounds as though it's the Moodle docs that are somewhat unclear/out of date here. The reference to "zip" files seems to be a bit of a red herring(!), because not only are the files not named '*zip', but with the default backup settings they don't even have a filename pattern that you can search for at all.
OK, so if backup files are stored within the "Course backup filearea" they get given utterly meaningless filenames, which makes it virtually impossible to find and identify the backup files, unless you have been able to query the database to find out the actual filenames (which you might not be able to do at the point where you really need those backup files).
As you say, setting up a specific folder in which to create the backups sounds like a much better system, as that way the backup files are in an identifiable location and they also have identifiable/helpful filenames.
Thanks again, your explanation makes the setup much clearer.