I think I know what you are talking about. In thai forest tradition this unconditioned mind is called with various names, right?
The Citta, The Dhamma, the knowing mind, everpresent citta, the Real Buddha, awareness, radiant mind etc.
In Zen they called it the Bodhi, Mind.
In Tibet they called it Rigpa, clear light mind, subtlest mind, mind like the sky etc.
Note that all I will say is just speculation and imagination, because I am not enlightened person.
It seems that at very high level of the path, one will access this knowing mind, and it will appear that this knowing mind is unconditioned, unborn, stable, eternal, undisturbed, and undying.
This contradict buddhist teaching that there is no permanent self. I believe that one must go beyond this knowing mind, see it as not self, empty, to finish the path.
Referring to above poster about two nibbana, I agree that the real unconditioned mind is realized when arahat has passed away and no future aggregates are produced.
Actually, this is the reasoning behind “three lifetimes dependent origination”. Even after liberated, Arahat still have volition, Consciousness, feeling, and six sense base, they dont immediately become unfeeling robot.
In my opinion, those teachers and tradition that teach about this knowing mind is not wrong. It is still a valid way in the path, and to achieve that is still quite a feat. But I believe one should go even beyond that to reach nibbana.
How can conditioned mind know the unconditioned? Well, by experience, and inference.
For example, if I enter jhana one time, then I know for sure that jhana truly exist. If I enter twice, then I know it is reproducible. If I constantly enter it, for sure I can teach it to others, because I know all the conditions necessary for it. Even though I am not in the state of entering jhana right now.
If I see there is no greed, hatred, and delusion inside me, and there is no condition that make them arise within me. If I know there is no condition for me to be reborn, then surely I know the unconditioned, and the path to it.