I thought my car was missing a part back there, but all Fiesta's are like that. The opening back there swallows anything small in the trunk, so I have to watch out for groceries falling in there. I stuffed a towel in the opening to make it better.
I finally took a look at mine and it's harder to see because my car isn't white, but I can now see the unpainted rear wall and the gap at the rear of the carpeted trunk shelf. It looks like anything smaller than a couple fingers width could drop down into the spare tire well.