The county recorder's office is the place to find the official claims location maps. These are individual to each claim location except in the few counties in California where they tax mining claims, in those counties the tax assessors office will have a map of claim locations, as John pointed out. Don't rely exclusively on the tax assessor map though as at least two of those California counties don't tax every claim nor are their maps necessarily accurate.
There is a lot of misunderstanding about claims markers. It depends on the State as to whether claims markers are required. In California, for instance, no staking or monuments are required to make a placer claim by aliquot part. Some (many) claims in California have never had corner markers.
In Arizona, California, Nevada and most other states there is no requirement to maintain a corner stake or monument once the claim has been located.
Read that again - THERE IS NO LEGAL REQUIREMENT TO MAINTAIN CLAIMS STAKES, MONUMENTS, OR MARKERS.
Relying on claims stakes to tell you if an active claim is located on any particular portion of land is foolish. Your requirement to know the status of the lands you prospect is limited to you as the prospector. It is not the claim owners fault if you prospect his claim without first researching and finding the land status at the county recorder's and the BLM.
Montana does require claim owners to check their stakes annually but that still doesn't account for cattle, elk, deer, gophers, prairie dogs and idiots knocking them down in the other 12 months of the year. No claims stakes does not equal no claim in any State.
The claim marker in question in the original post may or may not represent an active claim. That is determined in the usual way - research at the County Recorder and at the BLM.