I finally figured out what was causing the annoying rattle in the hatch area! The reverse light, which is affixed to the underside of the spoiler, is held on with two screws. One of the screws has come loose and fell out. Rattle rattle. Does anyone have the thread size and length of that screw, so I can just grab one at the hardware store?
It's a machine screw, with a shoulder. You'd want to get the correct screw, for the thread dia/pitch/length, and the shoulder is necessary to hold the plastic bezel. It's in an illustration on this page: MUDGUARD & SPOILER. For 2012 Toyota Prius Two HYBRID | Toyota Saying "refer to fig 81-17", and I'm a little stumped how to find that, so far.
Googling a bit more, pretty sure it's this: 90149-40012 High Mount Lamp Screw - Toyota (90149-40012) | Toyota Parts
I most always have a shopping list, lol. If you google images with the part number I posted above, it looks to be it. Took a pic of ours:
