questions on 2011 Prius 3 (first time owner)

not a prius mat, maybe they used it when putting dirty items in the back.

 

Looks like a generic mat to me. I have a mover's blanket that serves the same function, to protect the cargo area from getting dirty.

SCH-I535

 

Interesting concept. I'm mentally visualizing a giant vending machine into which you put money and out of which pops a Prius: Phenomenal

 

It's the front half of the Toyota Prius cargo mat set. You have it upside down. The nubs should face the carpet, and the "point" on the upper right in your picture would go in the corner near the tip of the umbrella.
I'll post a photo after I unpack the junk out of my Prius.