If you change the zoom of the document then you'll be asked about saving changes.
I think also that if auto saves have been made that will also trigger the question.
Does it happen if you open and then close within a few seconds, without changing the zoom or anything at all?
Thanks for the reply Trevor. I have queried my user and will report back on his findings.
It might also be a change in the printer. If the printer has been changed since the last time the file was saved it will register as a change in the file.