Trying to sort out my question, I came across this one, which is closely related, but it is not an exact duplicate.
According to those who say a woman must cover her hair, not just her head (those who hold Se'ar Ishah 'Ervah is Dath Moshe, in other words, MiDeOraitha the exposed hair of a married woman is considered nudity - this is very distinct from those who hold it is "proper" for a woman to cover her head to show she is married and the like), does this requirement hold in the home when other people are around, does it hold in the home when the window blinds are raised, and does it hold outside the home in a secluded, private place where no people are around?
In other words, is it treated as true 'Ervah, in which case so long as she can be sure that nobody can see, she can uncover her hair, but if someone could see, even in her own private home, she must not uncover her hair?