This problem is encountered when you search in the xml where you do not know the source behavior. I have come across in XSD search of Web Service WSDL. In particular tag prefix usage varies according to different web services. As an example;
To ignore the tag prefix in XPATH query; local-name() function must be used:
So your xpath query detached from tag prefix.
For further information about XPATH functions...
I use xpatherizernpp for XPATH check, a plug in for Notepad++