The Nine Walkers

Originally Posted by radagastly

This seems to be the only reference to Elrond from Gondor in the entire book. (If anyone can find any other, please let me know.) It is probably all that Denethor knew of him; his name. Add this to the fact that Minas Tirith was not their intended destination (except for Boromir and Aragorn) and it would have done little good for him to go, and a great deal of harm for him to leave.

Elrond was side by side with Isildur and Elendil during the Last Alliance's stand against Mordor 3,000 years before. For Denethor not to know enough lore to have heard an account of Elrond's doings during the second age against Sauron would be unthinkable. Elrond is every bit a legend as Elendil and Isildur in Middle Earth lore, and given that Denethor knows he is the wisest of all loremasters, ahead of Saruman even, he must have had other information bestowed about him, as one cannot just accept this view without a source of enormous credibility.