Oliver v. the U.S. (1984) (a) A place of business that is not open to the public is protected, so why aren’t open fields

[answerhappygod]

Oliver v. the U.S. (1984)
(a) A place of business that is not open to the public is
protected, so why aren’t open fields, since neither of them is
mentioned in the Fourth Amendment?
(b) The Fourth Amendment protects reasonable
expectations of privacy even though such expectations are not
mentioned in the language of the Amendment. Even if we granted that
open fields are not protected, does not put up a “no trespassing”
sign create a reasonable expectation of privacy?