This is why I think Cooper was drugged at Chick Fil A so that he would fall asleep, and also why I think he was placed in the trunk of the car so no one would notice him too early (i.e. before he died). I think the item being put in the car at lunchtime was actually Cooper Harris in his car seat, after being extracted from the trunk.
Remember the cop said he did not think the baby was in the car at 9am. Even though the paperwork confirms Cooper was put in his car seat in the car at ChickFilA at 8:55am.. yet baby was not in the car 5 mins later at 9am as they entered Home Depot car park/ I think after making a big fuss of putting the car seat in the car in ChickFilA car park, daddy snuck the car seat into the trunk before driving to Home Depot. Later at lunchtime, once he knew baby would have died - not only due to the heat but due to being smothered and suffercated in the trunk - he took out the car seat and put it into the car, hoping someone would spot dead baby in the car before he had to leave at 4:!5.. When noone spotted it, his whole plan went awry and he panicked..
He had never thought through what he would do if baby wasn't found by 4:15.. so he did not think to go to the daycare looking for his son (fatal mistake - obviously if he really thought he had dropped off his son that is where he would have gone before leaving Home Deport). Reports say he drove in the OPPOSITE direction to the way he would have gone home at first (probably in a panic, trying to work out what to do). He realized he could not cause a big fuss when he met his friends because his friends would see through the whole thing so he stopped off somewhere he would find lots of people - in a busy car park - so he had the ultimate audience for his performance..