I have never heard of this phenomenon of DCG and invisible/deleted folders before. In fact, I'm fairly certain that when I delete a campaign folder from the DCG folder, DCG can't access it again. Which makes me wonder if it's something particular to your Windows configuration. Do you have an automatic backup of your hard-drive and some synchronization thing happening?
Also, do you clean out your recycle bin? Or are there deleted files in there from months ago? (Although I just deleted an old campaign into the recycle bin, but I can't see it in my DCG folder using open menu when I select "Select DCG Campaign Folder".)
Sorry, I can't be more helpful, but it's nothing I've seen before.