I'm thinking more along the lines of it happened at the Anthony house because all the "supplies" were from the house/garage. (The baby blanket, the trash bags, the laundry bag, the special duct tape, the heart sticker.) I have a feeling Casey flew into a rage at Caylee for some reason because of the way the duct tape was not only over her mouth and nose, but wrapped repeatedly around her head and hair. I don't know if it was because Caylee did/said something to piss her off or that Casey wasn't going to be able to be with Tony overnight because of no babysitter. We've seen Casey in a rage and we know that what Casey wants, Casey gets, come heck or high water. I don't believe it was any sort of an accident, mainly because of the many layers of duct tape. I think it was violent.
So sorry to be graphic, but that's how I see it and I hope to heck I'm wrong.