This is too much paranoia. No they do not build features into their operating system for people who do not trust that they replaced the board for you, that is a waste of their time! Is there any reason to not believe them?
If this is that much of a concern to you, buy conformal coating pen like this http://www.all-spec.com/products/cw3300g.html and put it under the wifi card where they can't see it. If the green glob is still there when you get it back they didn't replace your board.