Microsoft Word documents will do this even on an open to update date fields and the last opened/edited field.
It's an annoyance, but proper.
I keep getting the "do you want to save the changes to xxxx.doc" when all I am doing is opening and closing the document with without making any changes to it, or even putting the cursor in the document. This is only happening in certain documents. Is there a box I have to check to turn this feature off in this document?