If we go back and look at the history of the 2nd/3rd
ages, we see that:
V. Tar-Meneldur Írimon (740–883)
"...Near the end of Tar-Meneldur's rule reports came in that evil began to stir in the east of Middle-earth (the first awakenings of Sauron, as was later learned), and Gil-galad requested the aid of Tar-Meneldur. Recognizing his son Aldarion was more aware of the troubles, and was a powerful leader of men, Meneldur resigned in favour of his son, far earlier than was expected."
VI. Tar-Aldarion Anardil (883–1075)
"...Yet he was the first of the Dúnedain to perceive the rising threat of Sauron, he gave needed aid to the Elven King Gil-galad and laid the foundation for the future alliance of the Elves and the Men of Númenor against Sauron. Without the foundations laid by Aldarion, his descendant Tar-Minastir could not have rescued Middle-earth from utter disaster in the first War of the Rings in the middle of the Second Age. His foresight enabled Númenor to construct a mighty navy without destroying her forest lands, and as King he ruled competently, keeping order at home and expanding Númenor's strength abroad."
- 883: Tar-Aldarion becomes sixth king of Númenor.
- 985: Death of Erendis, apparently by drowning.
- c. 1000: Sauron begins building Barad-dûr
- 1075: Tar-Ancalimë becomes the first Queen and seventh ruler of Númenor.
- c. 1200: Sauron seduces and deceives the Noldor in Eregion, but Gil-galad mistrusts him and refuses to work with him; the Númenóreans begin building permanent havens in Middle-earth at Lond Daer, Umbar, and other places
- 1280: Tar-Anárion becomes seventh king of Númenor.
- c. 1350: Celeborn and Galadriel together with their daughter Celebrían emigrate from Eregion to Lórien; Celebrimbor becomes lord of Eregion
- 1394: Tar-Súrion becomes eighth king of Númenor.
- c. 1500: the Noldor under Celebrimbor are instructed by Sauron, beginning of the forging of the Rings of Power
- 1566: Tar-Telperiën becomes the second Queen and tenth ruler of Númenor.
- c. 1590: The Three Rings are completed in Eregion.
- c. 1600: Forging of the One Ring; Barad-dûr completed; Celebrimbor begins fighting Sauron; Glorfindel is sent back to Middle-earth by the Valar
- 1693: War of the Elves and Sauron begins, the Three Rings are hidden
- 1695: Elrond sent to Eregion as lieutenant of Gil-galad
- 1697: Eregion destroyed, Elrond establishes the refuge of Rivendell, Celebrimbor dies, the gates of Moria are shut.
- 1699: Rivendell and Lindon besieged, Sauron overruns Eriador.
- 1700: Minastir sends a great navy to Lindon; Sauron defeated; Sauron's forces retreat from Eriador and the coasts
- 1731: Tar-Minastir becomes eleventh king of Númenor.
- c. 1800: Númenor begins establishing permanent settlements in Middle-earth, Sauron extends his power eastwards.
- 1869: Tar-Ciryatan becomes twelfth king of Númenor.
- 2029: Tar-Atanamir the Great becomes thirteenth king of Númenor but is hostile to the Valar. The Elendili or "Faithful" still receive the Elves in secret.
We can see that there was a clear relationship between
Numenor & the Noldor in Eregion &
Lindon.
XI. Tar-Minastir (1731–1869)
Tar-Minastir (S.A. 1474–1873, r. S.A. 1731–1869) was the eleventh ruler of Númenor. He was the son of Isilmo and succeeded his aunt, Queen Tar-Telperiën, when she died. He was the grandson of King Tar-Súrion. His name means "Tower-watcher", which perhaps signifies "The Vigilant". Sauron invaded Eriador from Calenardhon in S.A. 1695 and realms of Eregion fell. Sauron advanced to Lindon and Rivendell. In S.A. 1700 Tar-Minastir sent a navy under the command of Ciryatur, to save Lindon. Ciryatur's forces stopped Sauron's army at the river Gwathló; reinforcements from Lindon and Tharbad helped defeat Sauron at the Battle of Gwathló. Sauron retreated to Mordor in S.A. 1701. From around the 19th century in the S.A., Númenóreans began establishing permanent settlements in Middle-earth, including Umbar. He abdicated under pressure from his son in S.A. 1869 who succeeded him as Tar-Ciryatan.
Since Tar-Minastir was born right around the time when Sauron was able to deceive the smiths of Eregion, as
Annatar, and started giving counsel and lore and aiding in the forging of
the rings of power, there is little doubt he would have been exposed to the happenings in Middle Earth at the time. He would have had to have had some sort of relationship with Gil-Galad at the time, and surely some knowledge of the deeds of those days (forging of the rings/swift war surrounding their possesion), in order to be willing to commit a naval fleet. During Minastir's reign, after sending the naval fleet to defeat Sauron's forces, permanent settlements were starting to be made in Middle-Earth. In my mind, it makes perfect sense that now, of all times in Numenor's history, after having just aided the Noldor to defeat Sauron, that Numenor establishes these settlements for the purpose of "keeping an eye on Sauron". Moreover, I think Minastir would have had some sort of indication about who Sauron is, and some understanding about why Sauron was waging war with the Noldor in Eregion, and thus why they needed his aid.
So it MAY be able to be argued that some knowledge of the situation was known to Minastir. But it is a leap to say that he had direct knowledge regarding; a) the rings of power themself, b)
the One Ring. After Minastir's reign, little heed was paid by the
Numenoreans to the allies in Middle Earth and their transgressions, it is conceivable that after Minastir's reign, most communication between Numenor and the Noldor of Eregion ceased. There is little activity from Sauron from after the time he is defeated in Lindon.
In Unfinished Tales, "The History of Galadriel and Celeborn" states that Sauron and his bodyguard were all that was left of his original army. Gil-galad's power after the War was such that Sauron was unable to move out of Mordor for some time.
...From this time on, Sauron became known as the Dark Lord of Mordor. Sauron fortified Mordor and completed the Dark Tower of Barad-dûr, already centuries in the building. He distributed the remaining rings of the Seven and the Nine to lords of Dwarves and Men. Dwarves proved too resilient to bend to his will, but the Men were enslaved by Sauron as the Nazgûl, his most feared servants. Well aware of the strength of the Númenóreans after his earlier defeat, Sauron withdrew from the coasts and for a long time avoided directly challenging them, although he managed to use some of the Nine rings to snare three of their great lords.
Sauron regained control over most of the creatures that had served Morgoth in the First Age (such as Orcs and Trolls). Sauron also gained power over most of the Men in the East and the South, becoming their god-king.[31]
Toward the end of the Second Age, as the Númenóreans withdrew from the coasts after their people were divided by strife over the fear of death, Sauron began assailing their Middle-earth strongholds and assumed the titles of Lord of the Earth and King of Men.
It took a period of ~400-500 years from after Sauron's retreat, until the time we see the
ringwraiths appear around S. A. 2251. Undoubtedly this is the time period where he ensnared 3 Great
Numenorean lords. There must have been some communication and intelligence gathering from the permanent settlements at
Umbar and other places in the west of Middle Earth.
Another 1,000 years would roughly pass until the reign of Pharazon. After the first war of the rings, Numenor began to decline, slowly at first, but ultimately utterly. Since there are no guarantees that the royal court of Numenor at the time of Minastir knew anything about the rings of power, let alone
the One, it is difficult to fathom, that through Numenor's own strife and turmoil (With
Valinor etc.) any knowledge of Sauron and his relationship (With the rings) would be passed on accurately to Pharazon's time. Likely some of Sauron's treachery was already at hand within the Royal courts, long before Pharazon "captured" him. Numenor was dealing with internal strife for more than 1,000 years at the time of Sauron's capture. Undoubtedly Sauron's luring of Numenorean lords with his rings would have had some effect in Armenelos.
So although it doesn't seem likely that Pharazon; a) knew about the One, b) knew about what power it wielded, c) knew that Sauron had it, one thing does stick out as peculiar to me:
That Sauron, at the time of capture, would not have been subjected to a full search. Surely there would be a ceremonial (or forceful) removal of all possibly magically-imbued
items/
weapons etc. If I recall correctly, when
the ring meets Sauron's hand, the flame writing is visible, and surely this would have caught Pharazon's eye. How he may have been permitted to keep the One on during his "capture", baffles me. The only logical explanation to me is that Sauron concealed it during capture, which still seems in my mind to be difficult to explain.