Norotn Internet Security did not recognize Trojan.Zbot

I have a customer running Norton Internet Security 2014. They were informed by the local ISP that a government agency had alerted the ISP that this customer was infected by a dangerous piece of malware, "Game Over Zeus." My customer called me in a panic. The first thing I did was to run a full system scan with Norton Internet Security...the end result showing only 41 tracking cookies! ...no significant malware. I then installed Malwarebytes whose scan found 13 instances of Trojan.gen and Trojan.Zbot and removed the malware. I always tell my customers that Norton Internet Security is number one for blocking and removing malware. I am now concerned about how this infection could have slid past NIS. Can anyone enlighten me about this?