If the female caregiver concealed Wm's body during her secret drive that morning, she may have packaged him. He was tiny.
On the drive TO the airport, only the foster daughter was with her. The female caregiver COULD have stopped, retrieved a small bag, given the foster daughter a benign explanation and carried on.
In another case, a stepmother murdered her stepson, left his body in a suitcase in a field, retrieved it days later, and drove with the body and her own daughter, completely unaware, to the other side of the country, to heave it over a bridge.
So it's possible IMO that, en route to the sirport, with only the foster daughter as witness, one she was likely confident she could mind bend, she could have gathered Wm's remains from the first location and deposited it in another, closer to the airport. Even into a dumpster. Small parcel. Easily she could have lectured the foster daughter for 40 minutes, as was her later custom, about the ills of society and littering. Applauding her good self for seeing that debris and doing the right thing. Disassociating it.
I hope that the foster daughter has been able to unlock her memory vault from those ill-fated hours and days. IMO she was manipulated badly. But now holds the key.
JMO