Where are the stock-icon-names defined for the unity-panel-service indicators and notify-OSD?

notify-send uses icon from the /usr/share/icons/gnome/32x32 (& also /usr/share/notify-osd/icons/...)

devav2@devav2:/usr/share/icons/gnome/32x32$ ls 
actions     apps        devices  emotes     places
animations  categories  emblems  mimetypes  status

Above results are the different icons sets available for notify-send

devav2@devav2:/usr/share/icons/gnome/32x32$ ls emotes/
face-angel.png        face-sad.png         stock_smiley-13.png
face-angry.png        face-sick.png        stock_smiley-15.png
face-cool.png         face-smile-big.png   stock_smiley-18.png
face-crying.png       face-smile.png       stock_smiley-1.png
face-devilish.png     face-smirk.png       stock_smiley-22.png
face-embarrassed.png  face-surprise.png    stock_smiley-2.png
face-glasses.png      face-tired.png       stock_smiley-3.png
face-kiss.png         face-uncertain.png   stock_smiley-4.png
face-laugh.png        face-wink.png        stock_smiley-5.png
face-monkey.png       face-worried.png     stock_smiley-6.png
face-plain.png        stock_smiley-10.png  stock_smiley-7.png
face-raspberry.png    stock_smiley-11.png  stock_smiley-8.png

Example:

notify-send -i face-glasses "I am wearing glasses"
notify-send -i face-laugh "I am happy"

You can use the stock icons from other directories as well (devices,places, etc)

Apart from this you can use custom locations for icons

Example: notify-send -i /usr/share/pixmaps/gnome-irc.png "Icon Test"

I don't know if they're found in a list format somewhere in the hard disk, but there's a list on the web here:

https://specifications.freedesktop.org/icon-naming-spec/latest/ar01s04.html