Ah, ha! Thanks, guys. A good start to my day -- I've learned something new already.
So the round object is a resting place for the base of the jack. I had lifted the jack out just to clean the trunk, but I'm not sure I had the jack in the right place anyhow, so your answer is very helpful.
As for the clear lens, that's even more interesting. How clever to light the trunk this way! I presume it was only on the one side. Is that correct, John? And now the important follow-up question: can the small lens be replaced? If not, do new tail light lamp housing units that are sold include this item? I'll try to look that up. . . The only ones I can find in the online catalogs that show an image clearly enough are on the SLS website, and indeed it's quite obvious that the clear small lens is there. But the website only shows one side clearly enough.