His first big problem was the fact her car was there when police arrived, that eliminated most of the excuses he could use, if her car had been gone he had more options, perhaps the car was meant to vanish later.I think lawyers do a lot of "coaching" with their clients....lots of coaching. Like..."mate..,you were at home & asleep when Allison went missing".....just like in the Max Sica case where his alibi is "at home & asleep".
IF she had gone for a morning walk...one would hardly be calling in police to say she's missing after only an hour or so....
I've wracked my brains but can't think of one logical reason why he would call the police at that time.