From http://standards.freedesktop.org/icon-theme-spec/icon-theme-spec-latest.html

The lookup is done first in the current theme, and then recursively in each of the current theme's parents, and finally in the default theme called "hicolor" (implementations may add more default themes before "hicolor", but "hicolor" must be last)

Icons missing in the current theme but available in "hicolor" are not returned by QIcon:fromTheme