My SE Hatchback came with the cargo cover and 16 miles on it. No indication on the window sticker that it was an option either. I would call shenanigans on the dealer and keep pushing to get them to provide you with one (or the name of the previous owner to call adn harass to get it back).
With that said, you did buy a used car. Which might or might not come with everything the factory installed. Such is the beast.