I realize it's happened, but if you seriously can't go shooting without worrying about getting ambushed and murdered, well, I guess I'd say you need more to worry about. I'd bet you're more likely to get struck by lightning from a clear blue sky.
On the contrary it is no different than any other time. You have to be aware of your surroundings, the proverbial Condition Yellow.
What I am saying is that at a gravel pit it is easy to let your guard down because you are armed and shooting. We need to be aware that just because we are target shooting does not mean that we no longer possible targets of someone else. If I am anywhere and another person starts behaving strangely and is focused on me then I am going to do something about it: leave, let him know I am watching him as well, or demonstrate that I with a group.
Just because you are alert to POTENTIAL dangers does not mean you are scared or anxious. When you are displaying something that others are willing to kill for (guns, gold, wads of cash, etc.) then it is prudent to be more cautious.