Looks like you're on the right track. Found this post while searching opt_content[1].js

"I have seen this in many companies the last few days.
I want to clearify/explain this for all you mispleased users. The issue is related to adware, toolbar adware to be specific.

SEP and Norton will detect the file opt_content[1].js as adware. The Javascript file is an autoupdate script built to give you targeted ads and scareware to trick you into clicking on sites that the creater of the toolbar earn money on.

The problem for most of you users is that the source of the infection is not the script it self, but the greyware toolbar that you got installed the last time you download some bloatware. SEP/Norten removes the script, but the source (toolbar) keeps downloading it. Hence the repeat detections.The only way to remove the alert is to uninstall all unwanted toolbars. For SEP users, you can use application control to block toolbars completly,

I have seen this detection on serveral toolbars so there are multiple sources. It might be annoying that this happens, but after all the product is doing what it is ment to. Itīs up to you not to install bloated toolbars. Ufortently Symantec canīt block all toolbars as itīs not really illegal to create toolbars that the users willingly install. And the reason why this happens now, even though you migh have been running with the same toolbar for weeks, is that Symantec updated their definitions earlier this week and it wasnīt blocked until now."