Okay, so I found the case I've been referring to. Loic Rogers, in Montana. Note that in the first article, after discovering the boy's remains in the septic tank, the sheriff "declined" at that point to call it murder, but wondered how the lid was replaced if the child merely fell in accidentally by himself.
http://www.foxnews.com/story/2007/01/28/missing-montana-boy-3-found-dead-in-septic-tank/
and a few days later, death of Loic was ruled "accidental":
http://www.foxnews.com/story/2007/02/06/death-toddler-found-in-septic-tank-ruled-accidental/
If you've seen it once, it seems it could happen again.
http://www.foxnews.com/story/2007/01/28/missing-montana-boy-3-found-dead-in-septic-tank/
and a few days later, death of Loic was ruled "accidental":
http://www.foxnews.com/story/2007/02/06/death-toddler-found-in-septic-tank-ruled-accidental/
If you've seen it once, it seems it could happen again.