I am in the same situation as you, including disabling Wifi when putting the machine to sleep. I only noticed the automatic upgrade was happening when I woke the machine up and it suddenly started slurping down lots of bandwidth, and I could see it in my network activity indicator blinker. I was able to stop it before the download finished. Really unsettling.

I had every update option available turned off, including the supposedly-all-powerful "Check for updates".