Sunset was 16.33 and she went missing around 4, meaning it'd be just getting dark. Lights would certainly be going on and curtains closed. I've Streetview 'walked' from RD's flat to the top of Cow Lane which would seem to be the logical direction. Even in the middle of the day when the images appear to be taken, there are few people or occupied cars around and it's generally backs/sides of houses that face the road. One one side of Cow Lane it's mostly rear access so garages and brick walls, only the upstairs windows would be able to see the road. So I think it's a definite possibility that she *could* have made it that far without being seen.
Swanage School [High Street] finishes at 15:30, nurseries finish slightly earlier.... there WOULD have been a fair amount of school children in the area, not to mention mum's ferrying children about. I agree there would be a small window of opportunity of her not being seen on the 3-5 walk/jpg where she was heading... and it is STEEP