This missing icon problem comes up more often than it should and usually results in finger pointing between Norton and Google/Bing/Chrome or whatever browser the victim is using. The "what's your OS/version, browser/version, list of failing URLs and so on" goes on and on. There are lots of "reboot, remove/reinstall, is the Norton Toolbar turned in" questions.
I just spent 4 or more hours trying to use Norton's NSS tools to figure out why my safe web icons went away. After NSS tools, including "get support" didn't work, I actually called Comcast (who supplies me with NSS - they call it Constant Guard). I went through their tech support, who transferred me to network security, who helped me at least get NSS removed and reloaded but then sent me to Norton in case that didn't fix it. It didn't. After a lot of talking, the tech finally told me Norton's had a Safe Web server issue for a WEEK.
Any company that's had a week's worth of complaints about an issue they are working on and doesn't at LEAST put a notice on their website does not care a whit about customer satisfaction.