Of those three options, observable vs unobservable comes closest to what I am talking about. Known and unknown can only be talked about in reference to an entity or group of entities capable of knowing; observability has a connotation of objectiveness that "known" does not, i.e. what is observable can potentially be known even if it is not currently known by a given entity.
The definition of manifestation in your link is also centred around whether a (human) entity knows about the presence of another entity (e.g. ghost), but in this case the other entity hides its presence from the first entity. If we assume ghosts are real for argument's sake, then I would say the ghost is hidden rather than unmanifest.
When I think of "manifest" vs. "unmanifest" the first image that comes to mind is that of a ball perched at the top of a hill. While the ball is stationary at the top of the hill it has potential energy, which can be likened to unmanifest kinetic energy. When the ball is pushed and starts rolling down the hill its potential energy becomes manifested as kinetic energy.
In a cosmic context, the unmanifest could be regarded as pure potential: it is featureless and is therefore unbounded and unlimited, but by that very fact it is also incapable of "doing" anything while it remains unmanifest. The manifest (e.g. the universe) unfolds from the potential/unmanifest: it has distinct features and is thus bounded and limited, but by that very fact it is also capable of "doing" something. If we regard "manifest" and "unmanifest" as opposite sides of the same trade-off and think of the unmanifest as the potential/seed of the manifest, then we no longer have to worry about the impossibility of something coming from nothing.