Only your last example is valid for the case of a document falling into somebody's lap. Documents marked "for internal use only" are only enforceable through whatever contract kept them internal. Unless you are an employee or in some way responsible for keeping it "internal" then there is nothing to keep you from reading it. Writing "don't read this" on a document doesn't give that document any special powers.
In general, the same applies for classified documents. The US has no general "Official Secrets" act, so you can't prosecute just anybody for leaking - much less reading - classified documents. There are exceptions, but they must be narrowly defined.