elementary-xfce uses links to edit-find.svg for xfce4-appfinder (gtk-find) and gnome-search tool (system-search) but the 16px icon, being monochromatic and without bright highlights, disappears on a dark menu.
For xfce4-notes-plugin (link to tomboy's icon) it's the same issue.
Xfce's applications menu uses 16 px icons (at least in my 1024x768 screen) and so it's rather problematic to set these icons to monochrome just because they might be needed on the panel (note that the 'find' icon isn't needed there anyway, so it's another case).
Also there are GTK themes (like the one I use, MediterraneanNight) that set a dark menu while keeping the panel light gray so monochromatic icons don't work well in this case (this is what happens with the 'notes' icon, although there's not a white variant for it yet).
I agree it's a design choice and if one wants to use this icon theme one may have to set a custom panel colour.
But personally I think it's a troublesome choice because it just brings many inconsistencies (monochromatic icons only for apps that show on the tray and not for the rest, plus on the tray itself many apps are not covered by monochromatic icons so it's also inconsistent). I would stick with regular coloured icons for tray apps and with time provide a monochromatic choice (like Faenza does, IIRC).
If you want to stick with some monochromatic icons, you should work them in a way that they are visible in both light and dark menus.
I can only suggest, as my knowledge in computer graphics is not enough to contribute directly.
This has been fixed in a way since Greybird uses bright menus by default now (mixed themes don't work too well exactly for that reason, lots of menu-icons trouble).
As you can see in these screenshots from Xfce menu, (using gnome icon theme): http://ompldr.org/vZm43dg (using elementary-xfce icon theme): http://ompldr.org/vZm43dw
elementary-xfce uses links to edit-find.svg for xfce4-appfinder (gtk-find) and gnome-search tool (system-search) but the 16px icon, being monochromatic and without bright highlights, disappears on a dark menu. For xfce4-notes-plugin (link to tomboy's icon) it's the same issue.
Xfce's applications menu uses 16 px icons (at least in my 1024x768 screen) and so it's rather problematic to set these icons to monochrome just because they might be needed on the panel (note that the 'find' icon isn't needed there anyway, so it's another case). Also there are GTK themes (like the one I use, MediterraneanNight) that set a dark menu while keeping the panel light gray so monochromatic icons don't work well in this case (this is what happens with the 'notes' icon, although there's not a white variant for it yet). I agree it's a design choice and if one wants to use this icon theme one may have to set a custom panel colour. But personally I think it's a troublesome choice because it just brings many inconsistencies (monochromatic icons only for apps that show on the tray and not for the rest, plus on the tray itself many apps are not covered by monochromatic icons so it's also inconsistent). I would stick with regular coloured icons for tray apps and with time provide a monochromatic choice (like Faenza does, IIRC).
If you want to stick with some monochromatic icons, you should work them in a way that they are visible in both light and dark menus. I can only suggest, as my knowledge in computer graphics is not enough to contribute directly.