You can find the answer to this type of question at least in a legal way in court cases, mainly in the IBLA (Interior Board of Land Appeals)
cases. They have been mixed in past years but I think that if a claim is filed then no other claimant can locate on it without permission. All this changes if the primary claimant does not have the intent or is not working the claim commercially and the secondary claimant finds a valuable provable mineral deposit and shows capability and intent to mine it commercially. Best to stay off areas where claims are filed. On a placer claim you are only suppose to work the placer deposits which are deposits where the rock is no longer a hard vein or hard rock. This is usually characterized as an alluvial or eluvium
deposits but this may not always be the case. If the deposit is decomposed but has not significantly moved. Hard rock such as a pit mine obviously involves both the placer above and the hard rock below. A placer that has veins such as gold bearing ribbon schist that has decomposed may be able to be shoveled like an alluvial deposit that has moved but is still in place. Very grey areas.