In my Tundra, there's a flap on the bottom of the front grill that has holes and has screws that hold it into place. The plastic part you have looks just like the plastic part in the frame where the screws go. You'll have to get down there with a good light and see if you see anything not secure.....sorry but I'm not familiar with the Gen 2 enough. (All of my Prius have black clips and screws...nothing like that part.)
It's a fastener for one of the underside body panels. They tend to fall out in older cars. Nothing to worry about as the panels are secured with lots of fasteners. Just keep an eye out for anything that's getting too loose.
Assuming that all the plastic flaps are still present... 1. look for a plastic flap with an unoccupied hole in it. 2. look behind that flap at that position for an unoccupied square hole in sheet metal. Most of the fasteners go into round holes in the back piece, there are fewer that go into a square hole.
