My friend and I discovered a secret hive Ultra last night on the Dreadnaught.
It is accessed from inside the Cabal ship. You need at least two people.
Go inside towards the engine room area where you fight the tank. (think bros strike)
You need to clear the area of Cabal. There are two yellow health psions KILL THEM LAST. Their deaths open two doors near their respective locations.
Inside each door is a small room. In both you will find a small lit console. (The only thing lit in each room, can't miss it)
Both need to be activated by your ghost at the same time. Coordination is key! Also, the "deploy ghost" prompt does not appear while actively holding your ghost out.
Once this is done, the path to the bridge will be open. We couldn't find a way to get inside however their were a few cabal in there. Move past the bridge as if you were following the strike and you will meet a giant knight creatively named "Ultra Knight"
Slay him and you will get an emblem, he also has an exotic drop chance (I didn't get one, however the 3 of coins bonus I had active was no longer active after he died, so I believe it's possible)
Enjoy!
[b]Addendum[/b]: I see that my friend and I were not the first to discover this. That's not surprising. Many of you have had more time to search for secrets. I can't say that I have had as much as I would have liked myself. However, I felt it was important to share.
I've seen replies here that you don't need to kill the Psions last. That's great news!
I'm happy to see the excitement for this game. Let's keep things positive!