The "open fields" exception to the 4th Amendment is unfortunately very old-- it goes back at least as far as Hester v. United States in 1924, which held that the 4th Amendment didn't require a search warrant for a search of open land, even if that search involved a trespass.
That was before the modern situation of the exclusionary rule; such a situation, even if found to be contrary to the constitution, in that day would not have derailed a criminal prosecution like it does now. Instead, the government could be sued for the allegedly illegal search (and possibly, quite separately, for the trespass). This no longer happens.
I always find it amazing that ‘settled case law’ only goes back to say...1920. Anything older is just....old and forgotten.
I see no emanations from penumbras in my copy of Constitution. Damn, I always get the out-dated stuffs