Thousand Sons Tutelaries during 30k doesn’t make sense - huge plot hole

Practically every thousand sons sorcerer had a tutelary - as per the name, it was a part of their education and training in using the ways of the warp.

The tutelaries accompanied the thousand sons everywhere… even to Nikaea. Could the emperor / malcador not see the demons in his midst? Or are we to believe that these puny demons were the thousand sons’ best kept secret ever?