Oh whow, that was quick! Thanks for the answer!
So these items just disapear into nirwana, good to know better not to use rare items for outside decoration.
Does this apply to the inside of the house too?
Would be not so nice because I decorated with 8 Dibella statues I found/stole till now, the 5 glasses with insects and a floating stone of Barenziah.. I have no idea how that is possible but it does "hang" in the air. I dropped it with "R" because I wanted to put it on the bookrack. It did not fell on the floor but floats in the air.
Looks funny too so I left it there. I could not move it around like other items, by pressing and holding "E" it is back in the inventory again.
That stone of Barenziah is bugged, it floats in mid air for everyone. The items you place around your home will stay where you place them, those item coords are stored in your savegame file. And if something moves or falls on the floor when you enter your home, just reposition it again and it will stay put the second time.
Some items have been known to 'fall through' the cell, if that happens you're stuffed lol.