I'm with custom upholstered window cornice, the slot being part of how the cornice would have been attached to the little metal parts that anchor to the wall. There could also be a comforter in that long bag. Truth is, we don't know what it is!
I don't feel they would have taken a ride in the car to discuss things. Leaving two little children at home alone is one reason. If they had gotten up and could not find mommy or daddy, they might have started crying, screaming, or even tried to get outside to find them. (Speaking from parenting experience for over 28 years!).
Thanks Saint, for enlarging that photo. I knew it was old. I thought I had seen a long axe, but it was a metal squareing device. Anyway, there are plenty of "potential" murder devices in there, as does most anyone's garage. That handle you labeled looks like a broom or mop, not a shovel.
What kind of little car is that in there? From where the sofa is, they must not have used that car much. If it was 2004, like someone said, Nancy could have been pregnant with Bella, or around that time. Not that it makes any difference. Just a navel lint observation.