That reminds me of a similar experience I had about 10 years ago, with a twist.
About a half dozen of us hit our local shooting range and were mainly putting holes in the 50-yard and 100-yard targets. Then the RO called cease-fire and most of us went downrange to inspect our targets. When we returned and the "all clear" was given, I dropped a new mag in my rifle and pushed a round forward, took aim at about 16X at 100 yards, had my finger inside of the trigger guard and I literally had depressed the whole first stage of the trigger (and only a 1.5-lb pull mind you), when the back of man's brown Carhart jacket appeared in my scope, right in front of my target.
Another second, my round would have been downrange. Luckily I wasn't quicker than he was. I called an immediate cease-fire.
How this idiot wound up on the range and nobody saw him is beyond me. Brown Carhart, brown pants, brown back-stop to blend into.
The way he acted afterwards was also unnerving. I informed him that he was about a second away from catching a bullet in the back. He then admitted that he had walked onto the range from the SIDE, instead of from the benches like everyone else because it was a shorter walk from his car.
The next round of shots we all took, we all opened the bolts on our rifles and removed our mags as usual before walking downrange. This same guy not only took the mag out of his rifle, but he then immediately went ahead and loaded a fresh mag, closed bolt, rifle pointed downrange. I challenged him on this and told him that everyone was going downrange, and he might want to unload his rifle. He refused and, and in a very awkward and defiant tone he insisted he would sit there at that position and condition and wait for us to come back.
Needless to say, we did not leave the benches and walk downrange from this nut. We did pack all our gear and leave however.
Sometimes you just never know who or what is going to show up in your FOV. Luckily no one was hurt (although maybe butt-hurt) that day.
PT