Which icon is
indicator-session using? I want to tweak it manually but I didn't manage to find it out under
Is there a general way to find out which icon a certain application is using?
The icon is called system-devices-panel and there are indicating variants like system-devices-panel-alert. Their location is the status subfolder of your theme (or the fallback-themes) in
I would suggest you to create your own theme. Copy a existing one, have a look at Index.theme to see how a Icon Theme is defined and place your new icons in the right subfolders. You can easily start with just one new icon, the Index.theme file allows fallbacks and inherits missing icons.
Start with this Tutorial if you're interested in doing your own theme.