Were all of the trees marked pines? If they were then you are probably right about the forestry service. The Southern Pine Beetle it a huge problem here in the south, and they are trying to control it. They would not mark dead trees because the preditors of the southern pine beetle live in the trees after the tree is dead. It would mark either trees that have some brown needles or that are near infested trees. Another possibility is that it could be workers from a paper company, or some other company that depends on pine trees. The paper companies hate the SPB more than anyone because they leave black rings in the wood of the tree, and that makes it harder to process the wood for paper. I hope that helps things a little. Of course it could be something totally different, but I would think that it would be something along those lines.