Just to let you know that my sim owner (who is now... officially... my very favourite person in SL *g*) found my item using the Estate Script Debug feature. I am SO relieved - yayyyyy!
It appears that the item poofed to a completely arbitrary location, within another object on my land, which is why I couldn't see it visually, or when my items were highlighted. So Onix's answer was correct and it was worth talking to my sim owner :-) Cinnamon's answer was also correct, because if I'd had an object scanner I could probably have found it myself without bothering my sim owner.
Thank you all SO much for your help!
lushvelvet *I'm a very happy girl today* Nightfire