The thing about the Vocators is that not even the more knowing citizens of the Empire know about them, let alone someone on the other side of the known world. Other than things related to the empress, there's no secret in the Empire that is better kept. Just because something is old, does not make it more known. If you've kept something secret for several hundred years (while all that time, information technology has not changed much), you'll be pretty good at keeping that secret. If anything, people would assume they're an odd part of the Imperial Army, maybe some sort of elite vanguard.