That's irrelevant if you don't know the baby is in the car. Being married with children and having a wife who also works, we both have very busy schedules. I can see how this could happen on a very busy day where both parents may be off to a late start, however tragic. Having said that, I couldn't imagine having such a misunderstanding when it comes to one of our children. If my wife is taking the kids with her when she goes somewhere, I always verify that the children have gone with here before I leave to go anywhere after she's gone.
I'm guessing the mother put the baby in the car while it was parked and the father was still getting ready to leave for work, thinking he was going to drop the baby off at day care. The baby falls asleep, as babies often do in the car, and he never knew it was in there. The father gets in the car not knowing the wiser, thinking the mother is taking the baby to day care.
I really feel for those parents. They are probably feeling unimaginable grief and guilt over what happened to their little baby.