Boromir's search for Imladris

[Legate of Amon Lanc]

Okay, firstly, let me just repeat here what was already mentioned before - Imladris was a hidden valley. It was hidden in the way Mith mentioned (if you were looking for it, even if you knew roughly where it was, you had to find it in the labyrinth of rocks and valleys), and also in the way that it was a secret place. Most people really didn't know where it lay, and I daresay a commoner who even heard the name could not know whether it is in Eriador, Mirkwood or the mountains of Gondor. Think of all the "secret lands of XY" from our real world mythologies. Think of Gondolin. Rivendell was effectively something similar (smaller, of course, but originally coming from the same idea).

Denethor was a wise man and he would know about things like that more than anybody else in Gondor (as is even said), but still the exact location, for him, would be just "a land far away, beyond a thousand hills and mountains... blah blah blah" (I hope you get the point).

Originally Posted by The Mouth of Sauron

When Aragorn served Ecthelion in the guise of Thorongil, surely in conversation with him he might have mentioned Rivendell ?

Aragorn was in disguise himself. Nobody knew where he came from and where he disappeared to, and he certainly wouldn't boast about where he had been to, and the less about supposedly secret havens of the Elves. You could equally strongly expect him to talk about the Shire, which alone should show how improbable and illogical it would be.