Just to say, first, that I put a ticket in for this problem and the item has now reappeared in its original position.
Not sure, yet, if it was wound back by ZOS, or whether it somehow reappeared on its own?
The latter seems somewhat unlikely, as I had already tried reloading previously, to no avail, but still.
Anyway, what happened was that I was in my Sisters of the Sands apartment and my Music Box, Dreams of Yokuda disappeared.
I was moving a nightstand (that had the music box linked to it), to see what it would look like in a different position.
I then removed the music box, so I could place other things on the nightstand, to get a better idea.
As I did this, the game made something approaching the normal sound it makes, when something is removed and placed in your bags.
However, thinking about it, I did notice at the time that the noise sounded slightly off...
...slightly more alarming.
Having done that, I then undid it all again, via the Housing Editor.
That should have meant that the music box was re-linked to the nightstand, before the nightstand was then returned to its original position.
Instead, the music box did not reappear and the nightstand was returned to its original position, but without the music box on it.
I then checked my bags and the music box was not in them, either.
So, it had just disappeared entirely.

Just wanted to give you all an insight into what happened, as a heads up, so you can keep an eye out for this potential problem when you are decorating.