I'd only suggest that there is no evidence that she made it back into the house after she left to drop her car off. As it appears she went missing wearing the same clothes and shoes and with the same purse, it is something that cannot be ruled out. Her SIL cannot confirm she got back into her house. Her cousin says Sandra never called her back after making plans to discuss a visit once she knew more about time frames (recall SIL was running late and there were no phone calls exchanged between him and Sandra that day). I just think this possibility cannot be ruled out out-of-hand as no one can confirm she ever walked back into her house.
I know some have pointed to the lights being on as proof she left after dark, but I don't agree it's necessarily proof of anything. First, as noted in this picture, Sandra's living/kitchen area was lit by a set of recessed lights; more than one would necessarily expect for a tiny house, but given that there was so little floor space, it actually was a practical solution, eliminating the need for several tables with lamps or floor lamps. Less clutter for a small space.
These sorts of lights are usually on no more than one or two switches. "All the lights" may have been controlled by one switch.
Note also that all of the window light from the living area side of the house comes from one direction. Unless this wall faced south or maybe west, she may not have gotten much light in later afternoon, and perhaps she was in the habit of using the recessed lights during the day. Finally, it was not a sunny day. It was overcast and rained at times. Here's the weather for the area from about 2 - 5 pm that day.
1:54 PM 82 °F 69 °F 65 % SSW 17 mph 0 mph 29.81 in 0.0 in Light Rain
2:01 PM 82 °F 68 °F 62 % SSW 16 mph 0 mph 29.81 in 0.0 in T-Storm
2:16 PM 83 °F 69 °F 63 % S 13 mph 0 mph 29.80 in 0.0 in Mostly Cloudy
2:54 PM 78 °F 71 °F 79 % SW 20 mph 29 mph 29.81 in 0.0 in Light Rain
3:06 PM 78 °F 69 °F 74 % SW 14 mph 0 mph 29.80 in 0.0 in Light Rain with Thunder
3:21 PM 79 °F 70 °F 74 % SSW 8 mph 0 mph 29.79 in 0.0 in T-Storm
3:30 PM 80 °F 71 °F 74 % SSW 6 mph 0 mph 29.79 in 0.0 in Mostly Cloudy
3:54 PM 82 °F 72 °F 71 % SE 7 mph 0 mph 29.78 in 0.0 in Mostly Cloudy
4:54 PM 83 °F 69 °F 63 % S 16 mph 0 mph 29.78 in 0.0 in Mostly Cloudy