I believe the day was exacerbated by a sticky seatbelt that little Summer couldn't release herself.
It always struck me, in CW's account of the day, her specificity -- helped Summer with her seatbelt and she popped right up.
And what's that rule-- let sleeping babies lie (just not in hot cars)? Warm day at the lake, warm car, too warm zip up outfit.... if she left Summer in the car because she was sound asleep... carried in the groceries... helped Grandma carry hers in, help adjust her knee brace, lose track of time...
Truth: CW unbuckled the seatbelt.
Summer didn't pop up.
JMO
This seems like a good possibility I'd not considered before. As an East Tennessean, I know how cold the water is in the creeks and lakes and how sleepy children get after going swimming in them. That would account for warming up in the track suit and going to sleep.
The main problem I see with this is that the parents had to take care of the body. So, that's three adults and two children likely knowing. Since LE doesn't appear to think the parents were involved . . . I'm not sure everyone would have kept the secret this long.
My own "solution" came after Jose (was that the name of the man who stayed in the home while Don was working out West?) said his first thought was that somehow she'd gotten out the basement door. IIRC, he said she was just wild with running and impossible to catch and that was why the door was usually blocked with a couch (memory here, so may have been something else).
After reading his comment, I've gone with the above theory that Mom perhaps drank or took some drugs and fell asleep. That's why she and Don worked to protect her in order to keep the boys. If Summer got out the back door and did run wildly, she could have fallen and been badly injured or killed. We know the search parties continuously discussed how difficult the terrain was. So, I've thought of deep ravines, rocky outcroppings, caves or holes, old wells or cisterns that Summer could have fallen into. Even if she weren't killed or knocked unconscious immediately, a small child's voice is not likely to have been strong enough to have been heard any distance.
I've often wondered if Don should have waited until winter and had the fire departments in the area do a controlled burn so all the land would be exposed and they might find something that was hidden by the summer vegetation. I've considered that if it had been my child, I would probably have been compelled to make the land searchable in order to at least know whether she was there or not.