Looks like the decorative cover for the tow hook socket. There should be a hook along with the spare/emergency kit that threads into that hole so a towtruck can haul you out of a ditch without scratching up (much) more of your car. Probably just a bit of ice or mud blocking it from snapping in all the way.
Wow you are quick! I actually slide into something when backing up today that’s why it popped out.anywhere i can order a new one?
Very frequently rounded rectangles on the bumpers are covering shipping bolts. More rarely there are sensors. Prius Tie-Downs | RickRayfield at the bottom of this page, he discusses using them to attach Kayaks.
That part is specific to your car so a new replacement will have to come from Toyota. I haven't tried to buy factory parts online yet; simply haven't needed any for ours.
Genuine OEM Bumper & Components - Rear Parts for 2017 Toyota Prius Prime Plus - Olathe Toyota Parts Center I think Item 14 is the cover you want.
