Is it not known how the Marauders knew about Hope, the mutant baby. Unlike as with the purifiers, and as far as I know, there is no prior Marvel X-Men universe issues or plotlines mentioning that the Marauders knew about the baby before the releasing of Messiah Complex. The Marauders simply knew about Hope because that came as a direct idea from the X-Office.
According to the writers and editors of the X-Office in their X-Men Summit Notes (printed in the Messiah Complex Hard Cover Edition-Nov. 2008), after their Messiah Complex creative retreat in New York on Jan. 2007, they came with few vague ideas and characters that they share to the readers as follows:
The event (X-Men: The Messiah Complex) was divided into three acts told across four titles (Uncanny X-Men, X-Men, New X-Men and X-Factor) over three months (Nov.'07-Dec.'07-Jan.'08).
In Act One they did set the stage of all the conflict, bringing the players onstage and defining four "camps" they all fit in. One of those 4 camps was the Bad Mutants: Bad guys who want to find the baby to control it. These bad mutants are the Marauders. Their goal: to control the baby.