I recently moved out of Middle TN, and had lived in an adjacent county to Sumner County. There are a couple things that stand out to me.
This area of TN has undergone a massive building explosion in the last 5-6 years. Many homes are being built on what was former farm ground. This area is known for its limestone ground, and because of the limestone, there tend to be sinkholes. They can open up randomly and quickly, and many are unaware that they can open like that. I had a neighbor, who had one open up in his pasture, within 200 feet of his home, and they lost a calf down it.
Could Sebastian have found or seen a sinkhole and gone to explore it? Could he have fallen into one accidentally, especially if he was out in the dark with only a flashlight?
His mother talked about his love of animals. Could he have seen an animal, or anything else that potentially piqued his interest, that he wanted to go visit and hadn’t been allowed to go or been put off from going?
One thing that bothers me is both his mother and stepfather talked about how Sebastian doesn’t like to get dirty, which is common with children on the spectrum (have a grandson who has had occupational therapy for this). Would he have really wanted to go out without his shoes on, especially this time of year in TN, as it is mud season? That part of the state has been very wet this winter.