In the scene where Sandman is spotted by the cops, he goes and hides in the semi trailer full of sand. When the cops are walking along the trailer, you clearly see that the cover being used to cover the sand has several straps attached to the trailer holding it down. When the cop climbs the back of the trailer, you see him un-attach just one of the straps and you see the other straps are still attached. After unattaching that one strap, the cop throws the cover clean off the trailer onto the street. How did the happen?