I just noticed this on my wife's white MacBook. It's hard to see, but there's an outline of a rectangular...something...on the underside of the MacBook. It's not a crack and you can't feel it if you run your finger over it, but it looks weird. I noticed it when she was asking me if it seemed normal for it to get so hot. I thought it was normal, until I saw the outline where it felt hottest. I've never noticed it before, so I'm assuming it's new (though it's entirely possible I've just missed it up until now). Should I take it down to the Apple Store or is this normal?