I see no way the "threat" could have been in the vehicle on the way back from Alabama. Having the kids in the car would have complicated matters greatly. I think somebody entered the vehicle after the children exited and before Gail left the house in St. Ives. Based on my own logic only--nothing to cite here.
Pearl*, I hope it didn't sound as if I meant the threat was in the car all the way back from Alabama--no way this happened.
I understood that your theory is: the "threat" would have entered the car after she dropped the kids off. Thus when Gail was driving away from the house and down the street, it would explain why she looked straight ahead and didn't acknowledge Susie. It makes sense.