These are symbolic icons.
When using an icontheme it will adapt to the icon the theme provides
So, for example in the Papyrus icontheme search in the symbolic/actions/ for the icon: system-lock-screen-symbolic.svg.
I could be mistaken, I have not tested it, but it is a good candidate…
You can also install the gtk3-examples packages and use the icon browser app in /usr/share/applications to explore your current icon themes icons