I wonder if there are multiple doors - so for example she could get to her car via a garage or door without a step, but the door it seems she left by had a step or steps and wouldn’t be her usual path and so she wouldn’t have left that way on her own. My grandma’s house was like this - she had bad knees and used a cane but still drove, she just never used the front door because it had several steps and she didn’t trust herself when she was alone.
To answer the question about why someone at church was worried enough to call family, maybe she had a volunteer role that week and didn’t let anyone know she wouldn’t be there. I think if she missed a random Sunday at church maybe no one would be concerned, but if she missed a Sunday that she usually volunteered to read or be in the nursery or something, or had otherwise said she would be there like telling a friend she would see them tomorrow and then wasn’t there, someone would check on her and then contact family if she didn’t answer the phone or a text.