If people were coming and going from the house, to collect people maybe, then thats a clear opportunity to see the cars. Especially if you noticed them earlier in the morning, since you'd be driving right past them. Unless the cars had come and gone again, and maybe returned. Or maybe just left and it's an unfortunate red herring.
If the father left the house then that's a clear moment you'd notice the cars crowding the normally quiet street.
It's frustrating that the memory of the cars is only from a distance. At least from what we know from the media, that the police allow.