As a perfect mother of three, Allison could not leave her girls (if they were in the house) knowing that her husband went to bed and could not look after them. They could wake up and get up for a drink or something.
If the girls were not home, there was still a heave rain that night. I can't picture any sensible woman going out for walk in the dark, in the rain, alone. Certainly not a depressed woman.
The only reason why she could leave the house at this hour, imho, could be some late shopping, like milk, bread, tea, etc.. But even those things could wait.
And one more observation. If Allison was indeed watching footy, which I find a bit strange apart from other inconsistencies, why would she stop watching and go out? What time did the game end that night?
Actually, kiwi, you gave me another idea. GBC indeed could not know whether the neighbours called the police. He assumed that they could, because they surely had a good reason. So he tried to speed it up a little bit. May be he was not going to call the police that early, if not the screams.