I'm sorry. I totally disagree. If Cooper wasn't deceased and someone else noticed, than he simply had to start screaming that he forgot him. If Cooper was deceased, he would be in the same position he was when he "discovered" Cooper offsite. Just earlier in the day.
Ross clearly believed the police had zero reason to charge him with child endangerment after his son was found dead in his car, having been "forgotten" all day. He stated that emphatically. So what makes you think he would assume he would be charged with anything if Cooper was "discovered" while still alive?
He parked in a way that was close enough to other cars not to stand out, but also close enough that someone might have seen Cooper, even though he made sure there wasn't much visibility on one side. he knew form the get go that Cooper could be discovered at any time if someone came close enough to hear him cry, or whatever. All he had to do if that happened would be the same he did after he "found" Cooper - claim FBS.
Bottom line, he would have been anxious to know what was happening and filled with the urge to check. There was no reason make a detour to his car at that time and then casually toss the light bulbs that were so precious to him that he made sure to put them in his car at lunch, in such a careless manner and in a bizarre an unnatural way that kept him standing close and straight against the car.