Something came off inside the front bumper, Gen 3 2015 Prius

I believe that's your outside temperature sensor It should be clipped on to the center support piece of metal that runs from the upper radiator support to the bottom there's a hole where it sticks in and it reads the temperature of the air flowing through the grill if you will I believe this is how you get the outside temp on your display if you have that It should go back on its mount You should see the hole about 4 in from the bottom of the support up towards the sky

 

While you shouldn't have to do a complete disassembly, these 2010 (Gen 3) Repair Manual files will give you the general location where it should be attached.

Since it just came off of its bracket, and the wiring is not damaged, the sensor is still providing a temperature signal to the climate control system even if it is just hanging there.

 

Open the hood see the hood latch that the hoop drops into the clothes and latch the hood now come forward of that to the core support which runs across the top of your radiator and condenser now look straight down and you see a metal bar going from that core support down to the bottom frame of the car or lower core support whatever frame member that's called that little sensor goes into the little 5/16 or so whole that's about 4 in or 2 in above that bottom support but in that vertical member there's a hole right there for it reach your hand down there if you can if your plastic tray is missing on the bottom it's usually easier to lay down on the ground look up grab the piece push it in the little hole if the pieces damaged you can put some urethane glue insert it into its hole let that skin and dry you're done.