I don't think the length is the issue. Or rather, it's not what worries me. It's the snugness around the hips that gives it away, I think.
The saving grace is that most people don't go around wondering "hum, is that person wearing a diaper? No, well, what about that person?". Most people would never have it cross their mind unless something pretty obvious tipped them off. Even then, if they did notice, I suspect most people would stay quiet to be polite. So even if people notice, they won't say.
So I'd personally be worried most of the time unless I wore something really loose, like track pants.I just don't see any dress or skirt without a puffy slip really hiding anything. Then again, maybe I'm just paranoid. Oh well. :)