Interesting. I don't see it nearly so cut and dried (but rarely do, admittedly.)
There are several elements going on here, and as SG you have to make a decision for yourself what the answers are for your gameworld, the version of Mythic Europe and its history that your players occupy. There is no "right" or "wrong", only what "I" would do vs what "you" would do vs what "they" would do, etc.
First, in any world, the various branches of government vary in relative strength- in the USA, the judicial (supreme court), executive (aka presidential), and legislative (congress), tho' in theory balanced and interdependant, have at times been dominant or weaker, powerful or subserviant to the whims and decisions of the others. Also, any "law enforcement" agency can, at time, gain autonomous power, and, seeing itself as the saviour of all, possibly grow to abuse that power, or at least over-reach the scope they were designed to oversee. Just as House Bonisagus is, in theory, the "Guiding Light" of the Order, but varies in their view on the subject, so could the Quasitors, to one extent or another, decide for themselves that "they know best", and push, above board or under the table, for more power than what is suggested by the limited canon we are given. Your world, yours history to expand upon.
Further muddying the waters, it's quite possible that different "dynasties", different Praecos of the Q's, have had different policies/approaches, some more authoritarian and autonomous, and others more cooperative and interactive. That is, the difference between the J Edgar Hoover FBI, and the "kinder, gentler" one of later decades, and the current Praeco has inherited policies they must work with, or around. Perhaps they are not even aware of secret projects initiated in earlier times (see below.)
It would seem reasonable to some minds (both some players', and certainly some Quasitores'), that 7 years is far too long a time to wait to address a "clear and present danger" to the Order, to borrow a phrase. Just as, in cases of urgent necessity, the Q's can act with autonomy against a rogue wizard of the Order, or declare a Hedge Wizard or even other Mundane as an enemy of the Order and act against them, so (it can be argued, and reasonably assumed) could they declare one locale off limits. (Or not- up to the SG, but it's hardly unreasonable.)
And, as hinted above by others, a faction, sub-order or secret society within House Guernicus could arise to shoulder that responsibility as perceived, to do what must be done, even to bend the Code to breaking in order to save it. (Stranger and darker rationalizations occur daily in RL law enforcement.)
While the peripheral code usually only addresses generalizations such as "not endanger the Order", it does not attempt to address every instance or interpretation of that, but nor does it state that all magi have an equal vote to determine that, what does or does not constitute "endangerment". If that were the case, then every book suspected to contain Demonic Lore would have to be read and perused by every voting member, and then voted on. That is not the case, since reading such books is, de facto, understood to be endangering the Order. Can't let everyone read the forbidden knowledge just to agree that it is, indeed, forbidden. The Q's say it's bad, they burn it, done deal, next item on the agenda.
If the Q's determine that the very knowledge of Calebais is "endangerment", then, with that interpretation, just spreading that knowledge would be breaking the Code, and it is up to the Q's to stop that, even before it happens. (And remember, it doesn't take a consensus or Tribunal vote to reach that, only one arrogant element of House Guernicus, and the will to act on that self-certainty.)
How, then, to do this? And what to do if someone does stumble upon it?
Well, the are no unwritten rules than can be enforced, there is no "double secret probation". You can't have a doomsday device if no one knows about it. You need to keep someone out, but you don't want them to know they need to be kept out. Problematic.
The "sanctum" angle works little better than a Tribunal vote or decree, since the area is a bit large for a single magi's sanctum, and more to the point hardly appears to be one at face value. If a sanctum is kept a secret, hard to fault anyone who enters it. If you declare an entire covenant's worth of area a sanctum, it will raise questions, and perhaps invite visitors, and require that mage to have no other sanctum. Sticky.
There could exist a secret society/group, a cross between "The Men in Black" and those masked wahoos who for generations guarded the secret of The Mummy, who's (self-assigned?) mission is to keep such locations secret (and there could be many besides Calabais!)
First, hit the place with something akin to "The Shrouded Glen", to divert anyone searching for it. Then, by monitoring various locales via wards and such, a nearby "response team" could be notified to go out and deal with intruders. Tho' it would be difficult to imagine a Mage stationed to protect it, a nearby Quasitore could have (as a "Flaw") the duty to respond whenever needed by the secret society. And any Society of mage's worth its salt could have mundane agents located there, perhaps even someone/thing non-human specifically place or even "contracted" to that purpose. Lastly, there's a reason landmines are popular- magical traps tend to discourage many, and even dead magi tell no tales (usually).
But what if (as the current situation), despite all precautions, someone does stumble upon the knowledge?
If it's dire enough, anything is possible, from gentle reasoning and persuasion to strong-arm tactics, overt threats, blackmail, possibly even assassination as a last resort (it worked in The Mummy- or at least it was policy).
In a Saga I was in, one of the players went waaay around the bend, and it was decided their mage was possessed by demons (only explanation for their actions.) The Q's were going to come around, and take them away, and try an exorcism- only option short of a March (or a private War). What would happen next? Who knows, (and we never got that far). Maybe they'd be "disappeared", maybe get brainwiped, maybe a dishonorable discharge, maybe killed attempting to escape - anything's possible. A bit "men in black", a bit 1984/Stalinist Russia, but not all that unsimilar to this situation.
The question is, however- how dire is the knowledge? Is it worth killing for? Can they risk trusting in the cooperation of this mage? Or do they take more overt action, perhaps a powerful and special purpose PeMe "Calebais? What's a Calebais?" spell, or a MuMe "Calebais is not there" spell, or a ReMe "never think of Calebais again" spell. (Difficult to RP, but fun if it could be done.)
If you were sworn to uphold the Order, would you break the rules on occasion to save the Order? Some certainly would- history proves that, again and again.