I haven't had any floating plates, but my Sims tend to put empty plates and cups on top of the new kitchen shelf system from Island Living. And the only way I can see them (I play with walls down most of the time) is the green stink protruding through the floor of the bedroom upstairs. Took me a while before I figured out what was going on the first time.
While it probably isn't the issue here, but when looking for dirty dishes, be sure to check all possible shelves, including bookshelves.