Seriously don't read this if you don't know.
Epimethius: one of the founding grey knights is actually a fallen dark angel......and a psyker....and the authors of the HH DA books are given credit at the end of the book for "what should be obvious now" so hello Zahariel, good to see you again. Piss poor ending for you though you poor *******.
We also know.
Abbadon likely meets loken again at some point in the HH story line, he states that he "watched his wayward brother die" in reference to the founding members of the grey knights......so loken is one of them but is dead in 40k (no dah!)
There are others of the unforgiven that have fallen to chaos (corpulax was a consecrator)
And finally, the fallen don't really give a damn about keeping the DA's dirty little secret.
And what Abby got out of pandorax? Oh just a little bit of the gene seed used in the creation of the original grey knights.....wow! yeah that's a massive victory for him.