JMO -
.... Just thinking about a neighbour who might have been observing the front garden and had seen/noted that the FF had driven off at around 9 am, FF's destination and returning time would have been unknown to that neighbour!
.... Maybe he/she didn't see the FF depart, but would have been able to note that the Fosters' vehicle was missing! Again, when that vehicle would be returning would be unknown to that neighbour!
.... If William was observed later, to be in the front of FGM's house and his reason for being there was to greet the FF upon his return, it would be very risky for that neighbour to abduct William ... in light of the fact the FF could have returned at that very moment. (E.g.: For all the neighbour knew, the FF could have onlyjust driven as far away as the Corner Store to purchase the newspaper, etc,), which was only barely 5 mins away!
.... The FM has said that William was missing only for about 5 mins after jumping off the back deck roaring, and that she had gone around the side of the house calling out to him while frantically searching out there for him.
.... In that cul de sac, there is some distance from the front boundary of the FGM's house, so having grabbed William and then heading away with him:
.... In that short time, a neighbour would have likely been able to hear the FM calling out to William, and William would also have heard FM and thereby most likely screamed; that would surely have caused that neighbour to abort any abduction activity. MOO