So my random theory for the day, make of it what you will:
I wonder if she wasn't homeless, but close to it, maybe living in a hotel or something where it wouldn't go too unnoticed if you just left and never came back.
I've done some limited work with homeless shelters. Usually if you live at a shelter there is a time you have to be in by if you want a bed. And you can't store anything there, so people will have a backpack or duffel or something they carry with them to/from the shelter. The shelters pass out personal care items sometimes too - hairbrush, toothbrush, toothpaste, deodorant, etc. She didn't have any of that with her. There are organizations that provide clothing and shoes, unless maybe she just preferred slippers.
I think maybe she was trying to hitch a ride with the car that hit her, but they didn't see her. But I don't think she was walking around in those slippers for a super long distance. Wouldn't you get blisters after a while? There is a greasy smudge noted on her foot, but no blisters.
So what if she's living in a hotel, or crashing on the couch of some casual aquaintance, and she needed to go to the hospital? Pick your reason, she seems to have had a lot of health issues. Maybe she got a ride (or hitched) to the hospital. She didn't have a hospital bracelet on though, but she could have cut it off. But throwing on sweats and slippers to go to the ER isn't that abnormal. Maybe she left before being seen if the wait was too long. She was coming from somewhere, or going to somewhere late at night, and probably not walking that long given her slippers, her overall health, and the condition of her feet. Where else would one go from/to at that time of night dressed like that? It's not exactly an area bustling with activity from what I can tell.