/usr/share/applications/Terminal.desktop
/usr/share/applications/Thunar-bulk-rename.desktop
/usr/share/applications/Thunar.desktop
/usr/share/applications/mousepad.desktop
/usr/share/applications/thunar-settings.desktop
/usr/share/applications/xfce4-appfinder.desktop
/usr/share/applications/xfce4-mixer.desktop
/usr/share/applications/xfce4-notifyd-config.desktop

They showed up in KDE menu because OnlyShowIn=XFCE; line was absent in those files.

Other apps could be used, but xfce4-notifyd-config and xfce4-mixer may not be needed in KDE or GNOME since KDE and GNOME have their own replacements in my opinion.

This could be solved by specifying "OnlyShowIn=XFCE;" or "NotShowIn=KDE;"

I leave this problem onto your discretion.