Brake light lens cover fell off

It's sold and replaced as a sealed unit, so the chances of your finding one at the junkyard that has also come apart seem pretty slim to me.

iJDMtoy sells a replacement for $60 that also has a running light built in:
Smoke 3D LED Tube/9-LED 3rd Brake Light Assembly For Toyota Prius 4Runner Sienna

Otherwise, CarID
2010 Toyota Prius Custom & Factory 3rd Brake Lights – CARiD.com

Or Amazon

Replacement screws don't come with the lights:

 

Maybe it fell off, or maybe someone with a cracked brake light did an informal wrecker's yard acquisition? Strange, I can't imagine it just falling off.

The full assembly's over $200 :

Combination Lamps for 2010 Toyota Prius | Toyota Parts


Quoting Jim Carrey: "We got no food, we got no jobs, are pets' heads are falling off."

 

Oops - I posted the info for the third brake light, not the regular brake light. (I have it on my mind, since I just had to replace it). Same caveat still holds, though - no one sells just the covers, since they are permanently glued on (unless you live in AZ, apparently). I'd still suggest an aftermarket one over the Toyota one if you're buying new, though.

Replacement REPT730353 Tail Light | Auto Parts Warehouse

Getting just one new light, though, will look a bit odd until the new one gets aged enough to look like the original. If you can afford it, you might be better off getting a pair.

On the other hand, the same fender-bender that crunched my third brake light also whacked my driver's side tail light. I bought a set of two brand new tail light assemblies, so I will have a used good condition 2010 passenger light for sale sometime very soon. If you're interested in buying it, that would certainly be enough motivation to get me off my butt to install the new ones, even in this Georgia heat. Let me know if you want pics.