Firewall says Trojan, Antivirus says none, help please

I have Norton Internet Security from Comcast, and I happened to see 3 outgoing connections in the Firewall with the signature "System Infected: Backdoor.Trojan Activity 2".

I immediately ran a System Scan in the Antivirus, but it found nothing.

What do I do?  I am quite worried.

 

Edit (just found how): This is on a late 2013 iMac running Mavericks.  I used Liveupdate right before the scan.  I did not reboot first though, should I have?