JIMO my first thought; she got stuck, tried to get traction, realized she needed help, locked the baby up safely and went. It feels from the articles she knew the area, but thinking about the visit with friends… Just as an example, I have gone back many times to visit the family I lived with at Uni, in another city. At one time I had a great grasp of the area; after not going for 15 years, if my friends returned and I went to visit them, I could see me getting into a similar situation. Making it to their house, then leaving and having a brainwave. eg I know, that little waterfall in the meadow we used to go to, my Granddaughter will love it! She starts going the route she thinks she remembers, gets stuck. The reason she walks away from the road, not along it/stays with the car, is because she thinks she knows where she is. Since baby was fine, she cracked the windows, gave the baby a full bottle or sippy cup, locks up and starts to walk to a Ranger station/couple of houses/fire tower/Tribal hall, whatever she thought was nearby. I don’t see anything in the story about medical issues, she’s only 69, so if she wandered off the obvious answer to me is genuinely getting lost.
The only other thought I had is darker. Please correct me if I’m wrong, or not seeing it, but I cannot recall any mention of Mary’s key ring. If it’s gone and not located with her items, that would be very concerning to me. Keys are heavy, they get found. If you can find clothes, the keys should be in a pocket or with them. That’s my darker idea. Mary was accosted, when trying to dig out. Approached, frightened or uneasy she locked the door and threw the keys as far as she could, to keep the baby safe, then went with or ran from the hypothetical assailant
Either way, the SARs team certainly put a lot of effort in. I’m just sorry the couldn’t search for a bit longer. Sorry for the long post, I couldn’t think of a more concise way to explain my thought.