Hey come on guys take it easy!
This is an old farm /work truck, and if you have ever been in a rural area and saw one you would know, that most people look at them and respect them as a tool for a man to make a living, and leave anything and everything alone that is either inside, or hanging to the outside of the truck.
That old gun was a tool, and it was in the back of the truck, his truck, and that should be secured enough in my opinion, and no person has a right to reach into that bed to retrieve anything, such as guns, knives, axes, or hammers etc.
You might say, that you would have locked the gun in the truck, but people have been known to break into locked cars as fast as reaching into an open bed.
That said, it should always be reported in order to give the local LEO some idea as to who is working in the area, and with the hope that they will be on the lookout, so it doesn't happen so soon again!
The shame of it, is that the crackhead or panhandler, will trade it for a fix or a six pack, and it was a little more valuable to you.
And who knows, the new owner may just clean it up, put some cinnamon lams on it, and give it a good home that it deserves!