In Gen2 Prius It's a spacer that goes between front bumper cover and where radiator mounts... Seems the Gen3 one looks slightly different. If you look for a blow up diagram of parts in that area of the car you can see it's exact placement. Once an old Prius gets beat up enough they're problematic to keep in place and no harm in not putting it back, unless maybe if you live in really hot weather, in which case the radiator might be slightly less efficient.
In a gen 3 that goes up in the wiper tray If you look at your breakdown you'll see where it goes I think they might be two of them I have the wiper tray out of one right now It's probably never going back in and there's two little plastic pieces on each end of the wiper tray cover also but that's on the top plastic this goes inside separating the area where the wiper motor is from where water and debris might fall That's two little spaces where it clicks in look carefully you'll get it turn it around and you'll see.
In between the metal wiper tray and the plastic cowl cover toward the passenger side. Here's a video where they pull off the entire cover. I believe that part is near the passenger-side hood hinge. There's a similar part near the wiper motor, but that one has a hinge in the middle. You may be able to get to where it goes without pulling off the windshield wipers. Try pulling the plastic cowl cover extension on the passenger side first. Then pop out the plastic clips built into the forward edge. Hopefully you can pull the plastic cowl cover far enough forward don't he passenger side to get to where that goes.
You can see them here I think: Cowl panel & windshield glass for Toyota Prius XW30, 3 generation 01.2009 - 11.2011 - Toyota Car and Auto Spare Parts - Genuine Online Car Parts Catalogue - Amayama I thought it was maybe broken off, part of a larger piece, but maybe just unsnaps. While you're in there, put some 1/4" galvanized steel anti-rodent mesh over the cabin air intake.
