Let me think about this from a firearm enthusist for 60+ years. Shot gun on the seat of a truck, live round chambered, firearm not on safety. Something is not right here. No I know what you may be thinking. The dog is not that smart to take the safety off as well as pull the trigger. But then again, dogs are pretty smart these days.
Cross bolt safety, next to the trigger maybe. I could see a dog stepping on it, disengaging the safety, and a claw hooking the trigger
Not saying that’s what happened but I can see it being the case.