A mysterious icon has just shown up in my system tray, a white exclamation mark within a red star:
Nothing happens when clicked or right clicked (including no orange bar underneath like other icons here).
How would a user find out (on Ubuntu 18.04) what this is and what it's presumably trying (and failing) to warn about? This is just an example which I'm sure is easily solved, but in general are there any commands or tools that can display information about all current icons in the system tray?