It has happened to me before as well, and it doesn't seem to be just a Marriott problem. If I stay in a room with one, I will normally refuse the key. But even that doesn't always save me as I have been charged sometimes then as well.

I have noticed that some hotels have a "breakable" seal on the mini-bar. And as long as the seal isn't broken, they know I haven't been in there and don't usually charge.