“He had he’p or maybe a van, and did it several hours ~ to even days ~ earlier.”
That statement is ridiculous... A backpack sitting at the curb next to a security railing would no way go undetected for hours much less days. The only way it would go unnoticed is if there were a crowd of people there.
The thesis is that the cooker is in the bag and the bag is inside a black plastic bag. This is all perfectly normal given the heavy snows that hit Boston earlier ~ just trash here and there. These might have been set at the curb only to be picked up and tossed closer to nearby buildings during the sweep of debris before the race.
BTW, think of all possibilities ~ not just what your tiny mind can concoct ~ else you risk running down the middle aged white male terrorist route!