I had 5 licenses on my Norton Security account, and only used 3 of them. Of the 3, two of them kept on expiring and when I tried to activate the licenses again, I received a message that there were no more licenses remaining. On further investigation, I noticed all 4 of them were used by strangers. Somehow they were able to access my Norton account and steal the licenses.
There does not seem to be anything else that has been compromised on my PC's. I contacted Norton Support and they told me to change my password and delete the unknown PC's. I was told that they would investigate the problem after I assured them that I am VERY careful about the security on my PC's and never share my passwords with anyone.
I subsequently researched the problem on Google and learnt that Norton's network was hacked in 2006 and had their source code STOLEN. My guess is that my issue could be one of many more to come that are the result of the stolen code.