Icon theme doesn't always have to have it's own images for every icon needed, the themes can also define another theme to inherit from, and then all the missing icons will be pulled form that theme instead. Which is most likely what you are seeing here.
Originally Posted by jrogge
To find the original icon youa re looking for, take a look at the theme's index.theme -file. In the beginning you'll find a line like this:
This means that if an icon isn't found from ubuntu-mono-dark, the system should look if one is available in Humanity-Dark. And if not there, then gnome, and finally hicolor.
This also means that once you have found the correct icon and it's name, you don't have to edit any other theme, as soon as you create the icon in the theme you are editing the system will start using it instead of inheriting it from some other theme. (And also you don't actually need to edit an existing theme to change a few icons, you can instead make a new theme of your own, just add the icons you wish to change and then set the theme to inherit from ubuntu-mono-dark )