I found it in the spare tire area not connected to anything, but it looks like something that attaches to the trunk latch so you can open it from the inside? Just a guess...
It does look like a handle for closing the hatch. Was the previous owner short? But it looks like that’s the intended purpose .
Welcome to PriusChat! …a picture of the thing you're inquiring about might be useful? (…just a suggestion)
…hey! I resemble that remark! (…you should see this short-arsed Brit fitting the rear-shield into the rear hatch of my Gen II!)
Maybe we don’t want to watch. We might get a show out of the deal. Maybe more than we both bargain for.
Well, if I were Goldie Hawn (clad in a mini-skirt), that might in fact be true, and you might as they say, "…be able to see what I had for breakfast"¹ ¹ © Rowan & Martin's Laugh-In (from about 1970 (UK))
Here is the same thing on my Gen3, inside the added red circle. The whole plastic carrier assembly goes atop the spare tire: (oops, I should have snapped the jack handle properly into its recess before taking the picture.)