Constable has land owner permission to drive onto his rural land anytime as offenses have occurred there in the past. Land owner sells land but doesn't tell constable. Constable subsequently does one of his "checks" on the property and finds two folks with meth having a little pasture party. They, of course, have no permission from the land owner to be smoking meth on the land. They don't have standing to contest the "search" right?

I agree they have no standing. It would be no different than if they were hanging out in someone else's home doing meth--they can't complain about the party-crashing constable. Plus, it sounds like the open fields doctrine might apply.