On their website they are now saying the code was taken in 2006. It's good it only took them five or six years to notice, though they state they have teken steps since then to prevent it from happening again, so I bet they did know, but did not tell anybody they were exposed:

January 26, 2012, 12:50 PST

Symantec can confirm that a segment of its source code has been accessed. Upon investigation of the claims made by Anonymous regarding source code disclosure, Symantec believes that the disclosure was the result of a theft of source code that occurred in 2006. Since 2006, Symantec has instituted a number of policies and procedures to prevent a similar incident from occurring.

Furthermore, there are no indications that customer information has been impacted or exposed at this time.

The rest of the release is here

http://www.symantec.com/theme.jsp?th...us-code-claims

For a company that's business is doom and gloom, one would think they would have been all over this and not waited all this time for another group to announce there was an issue.