As regards “eavesdropping” on conversations between users of palantíri,
Tolkien makes some observations. (Pardon the pun.) Two users having a conversation, say, between
Saruman and
Sauron, or earlier between
Elendil and
Isildur, could not be eavesdropped or listened by another user,
except…
There were two stones in
Middle-earth that did permit “eavesdropping”.
The stone of
Osgiliath was the chief
palantír of
Gondor, and apparently all the lesser stones of Gondor –
Minas Ithil (
Minas Morgul),
Minas Anor (
Minas Tirith), and
Angrenost (
Isengard) – were subject to being monitored by that stone.
The stone of
Amon Sûl (
Weathertop) seems to have been the chief palantír of Middle-earth. Any communication involving the stone of Annúminas would seem to be subject to monitoring by the stone of Amon Sûl, and perhaps the stones of Gondor as well.
Communication between
Arnor and Gondor after the deaths of King Ondoher and his sons Artamir and
Faramir in battle with the Wainriders, when Arvedui of Arnor claimed the throne of Gondor as senior kinsman of the Anárioni (Heirs of Anárion) and on behalf of his of wife, Fíriel, Ondoher’s only surviving child, was probably conducted using the Stone of Amon Sûl (by that time removed to Fornost) and one of the lesser stones of Gondor, most likely that of Minas Anor, since the Stone of Osgiliath had been lost during Gondor’s Kin-strife.
The Stone of Elostirion, “Elendil’s Stone”, was not in accord with the other six palantíri of Middle-earth, but looked only to Avallónë in the Far West, even at the end of
the Third Age. This, I believe, was because its original purpose was so that Elendil’s father Amandil might remain in communication with his friends among the Eldar of Avallónë after Ar-Gimilzôr, twenty-third (of the twenty-five)
King of Númenor who was at covetous enmity with the Eldar, forbade their sailing from Avallónë to
Númenor; the other six stones were so that Amandil and his descendents might communicate in secret among one another in despite of the misrule of their kinsmen, the misguided rulers of late
Númenórean history.
Gildor and the other
Elves (apparently all
Noldor) whom
Frodo and his companions met in the Woody End, disrupting the approach of the
Nazgûl Khamûl, were returning from a pilgrimage to look into Elendil’s Stone, where they had apparently seen a vision of
Elbereth upon
Taniquetil. After the deaths of Elendil and Isildur, none of the Isildurioni (the Heirs of Isildur) ever looked into that Stone again. When
Elrond departed Middle-earth, he took the Stone of Elendil with him, forever severing all ties between Middle-earth and
Aman.
There is somewhere a comment by JRRT that
the Master Stone remained in Avallónë. If it possessed the same powers as the Stones of Amon Sûl and Osgiliath, we
might presume that it could ”eavesdrop” on any conversation between users of two stones.
The palantíri were made by
Fëanor in the deeps of Time, before the Rising of the Moon and Sun, so that one who used them could see with the sight of the
Eagles of
Manwë.
Denethor was aware of the true identity of “Thorondor”, his father’s chief captain beloved by all (save Denethor, apparently), before Thorondor overthrew the Captain of the Corsairs and burned their
ships in
Umbar, so also before Thorondor departed Gondor. But I think it was Sauron who pointed out to Denethor that the Heir of Isildur was approaching – both Sauron and Denethor seemed to have believed the Heir of Isildur was coming with the cavalry of
Rohan: neither suspected
Aragorn’s victory over the Corsairs at Pelargir, hence Denethor’s assumption that Pelargir had fallen and the
Men of Umbar, Gondor’s sworn enemies from ancient days, were sailing to join the assault upon Minas Tirith. While he could not overcome Denethor’s mind or purposes because
Denethor’s use of the palantír of Anor as its Ruling Steward was legitimate if redeless (and even “redeless” might be disputed), Sauron could control much of what Denethor saw using his palantír: thus Denethor lost all hope and fell into despair after Faramir’s apparently mortal wound when he saw the Black Ships of Umbar laboring slowly up the
Anduin toward Minas Tirith. (To be fair, Sauron might also have mistaken the Black Ships for his own Corsairs, though perhaps he knew the difference, and the
Witch-king’s predawn assault upon the Gates of the
City was a desperate attempt to take the City before the Dúnedain of the southern fiefs arrived to join the fray; but for myself, I believe neither Denethor nor Sauron knew of Aragorn’s success in the southern fiefs.)