I once was shootig a a USFS gravel pit when a couple of raggedy guys drove up in a beater truck. They did not look like shooters and they fooled around without making any signs of setting up for targets, etc. I decided not to go down range just then, and instead stuffed my 1911 into my waistband, pulled my Garand over within easy reach, made of show of checking chamber on both, and then went about "straightening things up." Just as I was running out of little things to do at the car and had decided to leave instead of hanging around with these yahoos, they scrambled back to their truck and hauled out. I waited a while to make sure they really cleared out, then finished my plinking. I was very alert on the drive out to make sure they were not lurking on any pullouts or side roads.
To this day I do not know if those fellows had anything bad up their sleeves, but I don't have any rational explanation for their behavior either. Most plinkers would be talking about their guns, asking questions about safety or how you're doing. Stay alert, stay armed and stay safe.