... on the side where it exists.
With the fix for https://sourceforge.net/p/xmlunit/bugs/46/ both XPaths point to the element owning the attribute. This can be more useful. If the attribute is present in the control document but not the test document, the controlXpath should be the one of the attribute while the testXpath would be the one of the element.
fixed with svn revision 580 - https://sourceforge.net/p/xmlunit/code/580/