I’m pretty sure that is a shotgun in the trunk. What I am also sure of is it’s dark. So, how long does it take, after one of the most infamous mass killings ever, for the DAMN POLICE TO SEARCH THE KILLERS CAR!!!!!!
I think his car was the first thing they searched and they found the shotgun in the trunk and just left it there. A witness saw the car doors had been opened and things in there strewn about.
When they entered the school they found the Bushmaster, thus this in the police dispatch:
9:55:25: Be advised, we have multiple weapons. One rifle and a shotgun
Since the shotgun wasn't used in the shooting they left it there and checked it later that night.