I noticed that when I opened my (finally valid) OPML file in Drummer it seemed to write it back to the disk with a bunch of thinks like and so forth.
I was mildly disconcerted that it had done this as I had been thinking about it as a viewer and I hadn't (to my knowledge) selected any kind of save option. I don't think I'd have minded if I had created the outline in Drummer in the first place or actually triggered a save.