Those white PVC corners are everywhere in Nevada. Mostly by the large mining companys protecting their future claims for minerals. As more land is removed from mineral entry, the older claims are still being registered to keep from being denied in the future. The funny thing is, is that the BLM and the USFS have been saying for many years that the use of 4" perforated pipe for marking corners is not an acceptable material for such use. It has more to do with enviromental concerns than legality.
So to reiterate, just because there are claim markers in an area you are looking at, does'nt mean that there is an active claim there. Thr LR2000 is way behind and only by diligent research can you be assured that the area is open.
There are areas out in Nevada on BLM land that are claimed. The claim owners put up No Trespassing signs to keep people out, but all public land is always available to everyone. They do that to keep out the occasional sightseer. A mining claim is for the minerals in that area only! It is not private land and the general public can explore that land at any time. Now a patented mining claim is a different story. That is deeded land which is private property.
What it boils down to is this. If someone has a claim and they are serious about working it, there will be evidence of recent work. With the price of gold recently, any who is an active miner will be working their claims vigorously.
Steve