Bubbles and mucous; when we were not there to observe the snake, it should not be shrugged off so immediately.
A vet is not a terrible thing, they are for the good of the animal, and it never hurts to be on the safe side when the drooling and spitting can very easily be signs of an advanced respiratory infection that can easily kill a snake.
Don't feed such large meals next time. Depending on the size of the animal, a just weaned rat is ample food for most ball pythons and there isn't such a large risk of injury, although it is not entirely depleted, it is minimized a bit.