WebDAV working group decided to call a URL without a resource
(e.g. 404 on GET) an "unmapped URL".
WebDAV got that wrong. The filesystem object at the end of a request
is not the resource -- it is only one representation of the resource.
The mapping is the resource and the resource always exists, which is
is why 404 Not Found does not imply that the resource does not exist,
but rather says that no current representation of the resource exists
at the time of the request.
WebDAV talks about it that way because, for the purpose of authoring,
it doesn't matter whether there is a distinction between the resource
and the current representation of that resource -- they become effectively
the same thing because WebDAV actions exist at an instant in time and
only work on resources that are directly authorable (one-to-one
relationship between representation and resource).