There is quite a long conversation between Sanguinius and the Night Haunter, both of whom are gifted (cursed actually) with foresight.

Both of them foresee Sanguinius dead, and both of them admit the Emperor was lying completely about chaos and what is to come.

Night Haunter has far better abilities than he does and states two things that Sanguinius cannot deny the truth of.

One: If they both can see the future, from moments from now till decades ahead, then time is like a book that the Emperor can read at will.

Two: It is very very suspect that they were "lost", but seemed to end up on worlds which suited their very apparent personality traits.

He also says he knows exactly what the Emperor's plan for the Emperium would be were it not for Horus, but the galaxy will forever burn in war so oh well.

He is very bitter about it, and his brother agrees that he's probably right but accepts that father knows best.

Was very interesting how he said what most of us have thought at one time or another.