I was inspecting my .xsession-errors for another reason, and I saw the following lines :
kio (KService*): WARNING: The desktop entry file .hidden/dirfilterplugin.desktop has Type=Service but is located under "apps" instead of "services"
kio (KService*): WARNING: Invalid Service : .hidden/dirfilterplugin.desktop
I checked, and it's seems to be indeed in the wrong directory :
$ locate dirfilterplugin.desktop
There is far more of this legacy crap - partly worked around via kde.eclass - to be cleaned up, when looking at the KDE desktop entry mess - even in KDE 4. But, beside the message, this is a non-issue from the user point of view. I'm not even sure right now, if there isn't a special case, where replacing the .hidden/ path with NoDisplay=true in the desktop entry might not work.
closing with same explanation as bug 224265