I am using Mavericks, the App Store preferences are set to automatically check for udates, but NOT to download or install...
The App Store application shows two updates available, namely system software (itunes& printer software) and an iMovie update. Via little snitch, I can see the storeagent process chewing up a considerable amount of bandwidth.... downloading what, I have no idea! this morning it has used almost 0.6Gb and is still going strong.
UPDATE: OK, just had a look at the App Store application again, verified that I had NOT clicked any of the update buttons. I then clicked the update button for the iMovie update and to my surprise the progress bar in App Store application shows that 600Mb of this update has already been downloaded! Not only that, now that I have proactively requested the iMovie update, activity under the storeagent process has not changed - ie it has been downloading all the time. I am pretty sure of this because if I now choose to pause the iMovie update, the storeagent process stops using up bandwidth.
My conclusion is that there is a bug in the App Store preferences which allows background downloads, in spite of the fact that "download newly available updates in the background" is NOT selected. It is odd that in my case only the iMovie update seems to get around this, whereas the itunes and printer software updates seem to be correctly prevented from running on the background.