I know it isn't wifi that would show up in the firewall. My old modems are still showing up on my account with no activity.... and they are both sitting in my garage pile of stuff to find homes or destroy. Zombie machine? never heard of that and will look into it. The P2P MS thing should still show up in the FW but I now see they have an option to only allow P2P on home network and not Internet so that is checked on all computers now.
I will get a hold of Comcast if it continues in March as there should be very minimal activity with no one here.
Edit: Spelling