I have always thought it was kind of weird that her purse was left at home, but her wallet was apparently taken. Not many girls/women I know *just* take their wallet when they leave the house. You might as well just take your purse, after all--it can store your keys, wallet, PLUS lip balm and lotion and whatever else you want. It's ultimately more convenient, I think, than carrying just a wallet, especially given that those days many people had bulky checkbooks in their wallets as well.
I wonder if it's possible Erin was abducted out of her home, and the person grabbed her wallet out of her purse rather than be burdened with the whole bag (which might have a weapon in it, after all). I'm assuming the house was buttoned up, though, or else that would have been mentioned in the police reports--or not.