I agree that the tossing of the lightbulbs into the car without leaning down even slightly looks unnatural.
But I am trying to figure out the motivation of that if he is guilty. Since he couldn't see him in that brief moment, it wouldn't determine if the baby was deceased or not. If he heard no sounds, that wouldn't really prove if he was alive or not since he could be asleep or unresponsive. Honestly it was such a brief moment of the door being open I'm not sure he could expect to hear anything anyway.
So what the motivation be if a guilty RH was unable to see or hear anything in that brief encounter at the car?
Maybe simply indecision. Wanted to check and see if he could determine whether Cooper was dead. But then couldn't bear to look: "I was afraid what he might look like." Remember, he said that. Maybe he was super stressed about it and wanted to see if he could detect anything but chickened out when it came to actually looking. Knew that if he heard no crying, Cooper might be dead. If he heard crying and anyone was near, he could simply decide at that moment to "make the discovery", even though Cooper would live.
Criminals usually obsess about their act and want to know what's going on, who's around, what is happening with the body. Is there evidence left behind. Etc. They sometimes can't stop themselves from returning to the scene, or getting involved, despite the risk.
But this dude, "I was afraid of what he might look like." He wanted to know, He didn't want to "see".