Clearly Titan, at the time iof the Heresy, was a "Black Ops" project of the Imperium and the Emporer himself. No one knew aboutm, including Horus, and every few people had access to it. Additionally Malcador, probably the great psyker of the time, second only to the Emporer himself, was using some type of latent abilities to camoflague this project from both public and private eye. So when the Origional Warriors arrive to find a fully stocked fortress-monastary(practice a full-seized fortress city in it's own right), it was quite the feat. As Lexicanum state........
Titan, through Malcador's mystical means, had been hidden away from the ravages of the Horus Heresy. When they arrived they found a Fortress-monastery already fully prepared and stocked with everything necessary to create a new army of Space Marines including supplies of gene-seed, which is suspected to have been taken from the Emperor directly, and all the necessary armaments. The fortress was also already populated with hundreds upon hundreds of suitable recruits from across the Galaxy. Some were raw and untrained, whilst others were selected in secret from among the ranks of loyalist Legions.
Therefore I only have two question. First why did Malcador go to such lengths to hide this Fortress from all knowledge? Daemonic Hunters or not, Emporer's command or not, it was one of hundreds of black projects they had. Why so different, what makes them special? Secondly how did they manage to keep such a fully operation center completely hidden from the naked eye?