It's nothing to do with namespaces - it has to do with which document (tree)
you are searching.

> Is there a way to force the current node for the key to be on 
> the 'data'
> document ?

In XSLT 1.0 the usual way is

<xsl:for-each select="$data">
  <xsl:value-of select="key(....

where $data is a global variable referring to the root of the "data"
document.

It's easier in 2.0, you can write

   $data/key('k', value)

or

   key('k', value, $data)

Michael Kay
http://www.saxonica.com/