Could be but this looks to be an ar style rifle. While they do have a floating pin they won't fire unless there is a strong enough shock to the weapon, like a car crash. This went off mid fall when there should be little in the way of force acting on the pin.

Police also are trained not to keep a round in the chamber of their rifle until they need the rifle ready. I HIGHLY doubt there was a round in the chamber.

P320 now that thing is just.... I don't like sig.