/usr/share/xfce4/panel-plugins/messenger.desktop is in xfce4-messenger-plugin 0.1.0-5.
This file is owned by root:root, with mode 0o644.
The actual contents of the file can be viewed below.
1 2 3 4 5 6 7 8 | [Xfce Panel]
Type=X-XFCE-PanelPlugin
Encoding=UTF-8
Name=Messenger
Comment=Notifies of DBus messages
Icon=xfce-mouse
X-XFCE-Exec=/usr/lib/xfce4-messenger-plugin/xfce4/panel-plugins/xfce4-messenger-plugin
X-XFCE-Unique=true
|