[Rumil]

Interesting question Mouth!

In Unfinished Tales, Christopher Tolkien speculated that Denethor might have been in touch with Saruman via the Palantir, if so this is even more peculiar.

However, Denethor was a pretty sharp customer, at least until convinced of the doom of Minas Tirith by Sauron. Maybe he detected some hint of Saruman's untrustworthiness, not proof enough to alert Theoden, but enough to make him uneasy about sending his son and heir to knock on the Wizard's door?

You'd think that Denethor could have used the palantir and discovered the orcs and halforcs at Isengard - proof enough to show Saruman's treachery. Also to find Rivendell and at least do a sketch map for Boromir.

The range of Denethor's palantir was around 500 miles, if I remember, so Rivendell is too far, but Orthanc is in range. However, in UT a mysterious process called 'cloaking' is mentioned in relation to the palantirs, whereby a place could be hidden from palantir viewing. No doubt Saruman had done this, perhaps this was enough to raise Denethor's suspicions?