For some reason xfce4-notifyd can't find its panel icons in 0.9.3-1.
Instead it shows a blank area in the panel.

If I downgrade to 0.7.3-1 without restarting xfce4-notifyd it shows a
panel icon on any interaction.

I initially assumed this was because the icons had been installed in a
different directory

/usr/share/icons/hicolor/symbolic/status

versus

/usr/share/icons/hicolor/scalable/status

But symlinking the contents of the former into the latter made no difference.