In-line:

For this document, Word decided for itself that my unofficial headings
were
official headings - at least it put them into the document map although I
didn't notice a new name in the list of styles (perhaps I just missed it).

Word doesn't decide anything for itself. It just does what you tell it.
Sometimes rather murkily anbd obliquely, it is true. But there is no magic
involved. It's up to you to take charge.

Actually, Word does decide, when the Doc Map is invoked, that certain
paragraphs (usually one-liners) must be headings, and therefore they need an
outline level. The Doc Map needs outline levels to function, otherwise it
wouldn't work. But it is not a good idea to rely on this, or to let it
happen.

Ways to control it:

Don't use the Document Map, use Outline View instead:
http://www.mvps.org/word/FAQs/Format...singOLView.htm

On switching into the Doc Map, look to see if a new AutoFormat has been
added to the Undo list. That's the application of outline levels. Undo it.
Remember to check for this every time, though it may not happen every time
(this workaround has only been tested on a Mac, and not thoroughly).

After the fact fixes:

Use the ResetPara command on the affected paragraphs to remove all direct
paragraph formatting and reset the para to only style-based formatting (the
doc map adds outline levels as direct formatting). ResetPara will not
affect direct character formatting (bold, etc) and the usual keyboard
shortcut is control-q (cmd-opt-q on a Mac).

Run this handy macro once posted by MVP Klaus Linke that resets the outline
level of every paragraph to the outline level that is set in the style.

Dim myPara As Paragraph
For Each myPara In ActiveDocument.Paragraphs
myPara.OutlineLevel = _
myPara.style.ParagraphFormat.OutlineLevel
Next myPara

If necessary, see:
What do I do with macros sent to me by other newsgroup readers to help me
out?
http://word.mvps.org/FAQs/MacrosVBA/CreateAMacro.htm
(Mac: http://word.mvps.org/MacWordNew/InstallMacroMac.htm)

Hope that helps,
DM