I had an incident a few years ago. I live rural on about 50 acres and our property has an electric gate at the entrance. My wife was just getting home and she called me from the gate saying "Some guys in a truck say they need to check the electric poles by the house". She said the truck had no markings on it. I told her to tell them to wait up by the gate (about 800 feet away from the house). As she pulled up to the house, I saw the truck right behind her with 3 guys in it. I did not hesitate to strap on my OWB gun and went out to see what was going on. I told my wife to go in the house and call the electric company while I "investigated". The guys said they needed to check all of the poles (there are 3 inside the the gate). They did have some sort of paperwork, but that was it. I told them they should go start back up by the gate (which they promptly did). My wife came outside with the phone after they drove back up by the gate. She had the electric company on the line. They said that they indeed were checking poles. I proceeded to give them an earful about the guys not having a company truck, no uniforms, no ID badges, and following my wife in after she told them to wait. The lady apologized and said they contract the work out.
In any case, the bottom line is, if someone comes onto my property (that is gated) unannounceded, or after they were told to wait, I am not taking any chances. These guys could have been anyone.
BTW, I almost always open carry while I am out on my land, but not if I am expecting someone like UPS or FedEx (and left the gate open for them).