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[schema error code] is only set when an element or attribute's local validity has been assessed (Validation Failure (Element) or (Attribute)). So no error code is set when no declaration or type can be found for an element or attribute, because in this case local validity is not assessed. But it seems to me that this should produce some error code if the context-determined declaration is mustFind. See the following: http://lists.w3.org/Archives/Public/www-xml-schema-comments/2002JulSep/0005.html
Clarification found here: http://lists.w3.org/Archives/Public/www-xml-schema-comments/2003JulSep/0080.htm