Never leave you bag on the ground or in an unlocked vehicle.
How ever it is okay to leave it on the back of your tailgate, have it run over by a truck and then returned to you.
I know this sucks Dan, I had it happpen to me at Riverside. The whole bag and a stash of discs. I will give you anything I have to help you replace your stuff. I even have 3 extra bags for you to choose from.