Oh they absolutely have, and the Dark Angels just kill anyone who learns. The Black Templars of the Ophilium Gulf crusade captured a Fallen once. The Dark Angels showed up, demanding the prisoner or they'll open fire. The Templars turn the man over. The Dark Angels then proceed to open fire anyone, destroying the Crusade and all the Templars aboard their ships.That's not really what bothers me about them. Their superhuman pride easily explains how they act. I still find them silly, but hey it's 40K. It's more of that these numbskulls have SOMEHOW kept their dirty business secret for 10,000 years even when tens of thousands of the Fallen have popped back into the galaxy since Caliban. You seriously want me to believe that not one of these many thousands of Fallen decided to tattletale on the DA to the Inquisition? What about modern DA and successor traitors?
Later when the Custodians captured a few fallen, including Cypher, on Terra, the Dark Angels sent in plasma gun armed kill teams, and killed dozens of Custodians and Fallen before detonating themselves with plasma bombs so there would be no trace of who attacked the palace .