We're going to get some paranoid Marines out of this, aren't we?
"Well, about 300 years after they were founded, they just disappeared off the fucking map. They were fleet based, so it wasn't like they had a home-world top search for clues. Afterwards, we've seen evidence they still exist: shipments to resupply a chapter are disappear, Marines getting told that another group of similarly garbed Astartes already grabbed their stock of potential recruits a few weeks prior. From what we've gathered, their tactics are extreme and harsh. They come out of nowhere, apply for more force than is strictly needed, and then vanish. We estimate they've been growing their ranks, and currently are splint into three to four groups roughly chapter strength, given the yardstick the Codex gives us."
"Brother-Captain, if these people exist, why haven't we destroyed them?"
"Because They're extremely loyal to the Imperium, from what we could tell. They only attack clear enemies of man, and shun outsiders so intently that there's no way they've been corrupted. That and we have no fucking idea where they are."