This is a game, and we don't talk reality and common sense, please. With half an eye open you can still identify a suspicious person moving in the disguise of an extra barrel in your own house even when that barrel is out of your "cone of sight".
Okay, point taken on the whole evoking common sense thing. You're right that it's a game and common sense doesn't apply.
Still, invisibility skills being an effortless "easy button" to infinite wealth isn't exactly what I'd call good game design.
