I unlocked one without knowing I would have. I had a bunch of points in the supression tree. I used a point refund item to respec and I got a perk reward for doing so. I only know it must have been a hidden one because I had recently made a thorough looking-over of the knowable assassin heroic deeds I hadn't yet completed and I knew that that hadn't been one of them.

Is there even hidden perks? The perks that can be unlocked per class are shown in the Heroic Deeds tab as much as the global ones. They include info on how to unlock, even on the Perk selection list have the method to unlock them.

Unless it's some perks that doesn't show anywhere that I'm not ware about.