If your covenant has been sitting in a secluded spot minding it's own business for 10 years or so, and suddenly a nobleman arrives with documentation indicating that the covenant's land has been granted to him by the King and he would like the covenant to either swear loyalty to him or vacate the premises- would defending the covenant be considered interference with the mundanes?
Only if you get caught and don't have enough friends at Tribunal. 
Also, you could defend it without obvious magic.
Ask the noble if he freely consents to taking over the land and all the responsibilities it holds, including protecting it from the faerie horrors of the neighbourhood. Ask him, just to make sure, that he hasn't heard of the curse of the castle. If he still wants it, pack your lab equipment off and leave laughing hysterically "Free! Free at last!" and then proceed to use Veil of Invisibility to play tricks on him until he leaves.
The rule against interfering in mundane affairs is enforced differently in different Tribunals, and is influenced by the attitude of particular Houses. So, in Normandy, where magi and nobles live in close proximity, the bar for "interfering" is much higher than in, say, Provence. In Novgorod, there are so few Quaesitors and so much space that magi can do almost anything, especially if they're willing to clean up afterwards. Jerbiton supports involvement with mundane authorities, so Tribunals where they have more influence are more likely to forgive this behavior.
As others have noted, it all has a lot to do with the politics of your situation and, implicitly, in the stories you all want to tell. If you want to tell a story in which the magi defend the covenant against the noble, but you DON'T want to get bogged down with Hermetic politics when the magi are challenged for doing so, then it's easy to invent a reason why that challenge never happens.
I'm thinking of areas where there are already mundane land disputes- for example if you were in Novgorod and the newly conquering Mongols came in and laid claim to your land, or in Hibernia where the English king has granted land to a nobleman... or perhaps in Iberia where a newly conquering king is granting land to a knight...
There are all sorts of potential responses to this, and some of them depend on which Tribunal the magi are in.
The nobleman shows up and "would like" them to swear or depart? Or the nobleman demands one or the other?
If he's coming on soft, there's a lot of negotiation to be done. He wants something - granting him that may well be in bounds and not interfering. Possibly he can be bribed, dealt with cautiously, or told to go away politely or impolitely. If he's coming on hard, it's still likely that they can come to an accord that does not require fealty. Military service might be granted without fealty objectionable to some Tribunals. Maybe he wants magic items on a steady basis - that's much harder to deal with, but isn't impossible.
The magi have only been on this spot for ten or so years? Then they can't exactly claim that they have been since time immemorial.
Of course, maybe he wants magical allies - that could be defensible.
He may want something the magi are sitting on. Or just taxes.
I'd find out what it is he's after, and go from there.
It's not a bad idea to authenticate the royal decree, too - he may be conning them. If it's real, possibly the king was unaware the site was inhabited ... by magi who have just bribed him with a magic device... (which will get them in trouble if the king shifts this sort of behavior to other covenants). If the document is not real, the nobleman could himself be in great trouble for using the king's name.
The point of not interfering with mundanes is to keep magi's lives unbothered. You know, bringing ruin upon my sodalis. An argument could be made that rolling over whenever anyone with a paper tells you to swear loyalty to him is going to encourage further annoyances of the same type.
In a perfect case, I say develop and use some mentem magic to figure out what's up and then subtly twist the situation so you are not only not disturbed but also no fingers can be pointed to the covenant as the architects of its own good fortune.
Depends on the specifics of this place known as "spot they've been at for 10 years." Unless it's a castle sitting in the middle of an open field ten miles away from one of the noble's fiefs (in which case you absolutely deserve the demand to swear fealty) you can probably have somebody say something akin to "you know how politics work, give us a week to bicker and then come back and we'll probably swear fealty to avoid war" in a more sly and appropriate way (I'll admit, I don't know a thing about how to say things with double-meanings that actually translate well) and then while he's off rubbing his hands together and possibly preparing to attack you, you (procure some vis and) set up things to keep the noble in question from reaching you. You're in a forest? Suddenly the foliage is too thick for a large group of armed combatants to get through. You're on a mountain? Sorry bro, there's a sudden blizzard around the whole mountain. Except the top, but you don't know that. Come back next time we feel like suffering your presence. You live in a cave(rn system)? Who the hell makes a covenant in a cave(rn system)? Anyway, just construct yourself a new opening with magic, cave in your previous one. Or better yet, make an illusion of a cave-in at your previous one. Either will work.
Long story short, pretend you're not there anymore or can't be reached. If he persists? It's not like you shot lightning at him! He's the idiot who decided hiking up a mountain in a blizzard was a good idea. Assuming your Tribunal doesn't see it that way, well, weigh the value of what you have. If you think you can make it just as well elsewhere, move.
Ahh, I'm so bored I'll answer this like a lawyer since the code is a quasi-criminal statute which is the in-game intention of the designers who unfortunately did not provide any material proofs for the various offences! I am going to put to this thread that the following elements will provide the material proofs to found the offence of interference with a mundane:
- A voluntary act;
- that hinders, obstructs or impedes a mundane person;
- with intent to hinder, obstruct or impede.
Elements 1-2 are the external elements of the offence with 3 being the mental element. Element 1 is made out (they built a covenant), element 2 is made out (its in the way of a mundane lord). However there may be no intent since the place might have been vacant at the time they committed 1 and 2, 10 years ago. All three elements must be committed contemporaneously so the covenant could be in the clear. However if they possessed knowledge that the mundane lord had good claim to the land at the time they committed 1-2 10 years ago, than their intent to hinder, obstruct or impede could be imputed by their knowledge of this fact.
PS. Forgot the last bit, Ruin on the Sodales. I will have to think about it some more, but clearly that element is not present in the facts. Even if the mundane lord decided to kill, maim, imprison or a member of the order you still lack element 3. Come to think of it, if you apply the contemporaneity rule that would considerably narrow the application of this offence.
Lord Shag: The Code means whatever the Presiding Quaesitor and the majority of the voters at Tribunal want it to mean. Modern legal theory and procedure don't apply.
Anyway, back to the original question. As has been said above, it depends on your Tribunal, but under the circumstances you mention, the king doesn't have any authority to grant the covenant lands to that noble that the covenant has an obligation to respect. The proper response is to tell him "no," possibly in very rude language. If he brings knights to make his demand, the proper response is to burn him and all of his men to ash. Outside of places like Stonehenge (where the Order has to come to an accommodation with the King) or the Greater Alps (where, depending on the book you're using for a source, covenants are supposed to stay well out of mundane sight) sodales will not give you any grief over doing this; you're defending what's yours and sending a strong message to the rest of the kingdom's nobility not to mess with the Order of Hermes.
Lords of Men: Chapter 4 has more info on the subject.
How dare I intrude upon your ruminations Ramidel. My profuse apologies. Are you saying that identifying facts that could form an offence is without contemporary precedent? I did not completely gather what you were saying.
Identifying facts that "could form an offence" is not out of the mythic paradigm, but it's not how the Order of Hermes forms its law, and there are examples of violations of the rule against interfering with mundanes that do not fit the fact pattern you describe. What actually happens is that the Tribunal decides whether, in the judgment of its members, the law is violated. Facts only matter to the extent that the voters care about them.
For example, the rule against the creation of precious metals is a Peripheral Code ruling at the Tribunal level (implemented individually by several Tribunals) that is considered to fall under the proscription against interfering with mundanes. In some Tribunals, the metal doesn't even have to be circulated for this ban to apply (Lords of Men 37), despite the fact that the metal does not "hinder, obstruct or impede a mundane person" and was never intended to. In other Tribunals, the circulation of the metal does not have to cause any provable harm to mundanes or to the Order.
Hibernia - if the English want it, they can conquer it just like any other land they're seizing.
Novgorod - Just because the Mongols are coming doesn't mean they own the land. On the other hand, these are the guys known for "they made a desert and called it peace". Can you match them in ruthlessness?
Iberia - same as Hibernia.
In each case, the Magi are within their rights to tell the would-be noble to piss off, and to roast him if he persists. (Of course, this might lead to reprisal attempts, ultimately backed by either the most potent relics or the mightiest spirits that the invading faction can get hold of.)
The thing is, aside from England, where William the Bastard declared that all of the land was his, a king doesn't necessarily have claim over any given tract of land. If the magi have an allodial claim to their holdings, and can continue to thwart would-be conquerors, they have no legal obligation to give over to anyone. (And if they have any neighbors with allodial claims, said neighbors are going to be uneasy at the thought of anyone trampling on their rights.)
-Albert
Heh, brilliant 
The act in question isn't the founding of the covenant, however, but rather how to deal with the noble in a way that doesn't interfere with mundanes.
Swearing fealty to the noble is clearly not an option, and I presume the magi don't want to give him the land and pack up and leave. They must hence either bargain with him, or fight with him, or manipulate him to let them be.
Manipulation is the most interesting; I REALLY like darkwing's suggestion above, but I would not offer him the land even in jest. Instead, arrange for him to spend some time in the covenant or its environs and arrange for all sorts of supernatural trouble to brush against him, with the PCs kindly aiding him against it while casually mentioning that he'd be expected to handle those kinds of things once he's lord of the place. And yes, definitely mention the Curse that falls on any that lords over the place, and the terrible monsters that he needs to protect the villagers from, and that he must never, never let anyone into the cellar because there is a demon bound there that will desolate the whole land if set loose, and...
Thank you Lord_Shag, I really appreciate that you showed how a modern approach would tackle this issue.
Don't mind Ramidel, it is always good to show another way to look at things.
On the topic itself, I don't have much to add. The "canon" answer is really depending on the Tribunal, but even more so of the covenant influence and reputation - baring the most obvious display of magic in front of a large mundane crowd, since judgement are more a matter of politics and influence than a clear cut, logical approach.
But the most important point - for me at least - is the one from Doctorcomics: what kind of stories your players want to have ? Mundane politics & intrigue ? War of a few magus vs large army ? Hermetic law and powerplay ? Once you know that, then you make sure that the local laws/rules/traditions will help you wave the story you want.
From a meta perspective...
Such a story may not be appropriate for some sagas. If the players have chosen all of the hooks of the covenant and/or agreed to SG proposed hooks that don't cover this occurrence, and if the players don't have story flaws that could trigger this, then I wouldn't play the story, and I'd present my misgivings about the story to the SG.
Or the covenant does interfere, and the story focuses on the Hermetic repercussions, rather the mundane aspect of things.
From the SG perspective, I would add a twist. Have a politically rival Covenant ( even one that the player Covenant might not know is a rival) work behind the scenes with the Noble in question. Perhaps he is a companion or at least a pet of the rival. They are using this mundane pressure to get something from the player Covenant. And it makes it harder to deal with the Noble easily.
While the idea that the Code means whatever the Magi at Tribunal makes a lot of sense considering the period and the nature of the Order, it's emphatically not the canon interpretation. The sourcebooks suggest there are considerable case law precedents in each regional Tribunal and there are certainly Quaesitors with a quasi-professional status and function, empowered to keep the Tribunal acting in accordance with established law.
Who says? The King makes the law of the land. Or should His Highness be governed by Hermetic Law? How about Roman Law? Well, the Romans have been gone for a long time. Church Law? Good luck appealing to the Church, you bunch of heretical witches.
That's the crux of the matter. Mundane authorities act as they see fit, and covenants must find a way to coexist. Outsiders don't have a whole lot of rights, other than those they can successfully assert for themselves.
It depends where you are, but in a lot of places, no, the King doesn't make the law. If he's powerful enough he can ignore it, sometimes, because bringing him to task is difficult, but there are many kingdoms where the law is above the king. Hungary and England come to mind immediately. In Scotland the King is just the ruler of the subkings, and they have a heap of rights on their own land. In the Byzantine Empire, the ruler's not above the law...
So, it depends where you are.
A better way for me to have said it would have been "The King and his court/curia make the law, under constraints of societal interpretation of customary law and abstract justice, and also under the practical constraints of power relationships and enforcement potential".
In the situation we're discussing, it means the same. There's no abstract law or Supreme Court for the Covenant to appeal to if they disagree with the King. Mundane authorities as a whole are in charge of the mundane world and the Magi can either participate, hide, or resist.
I think you'll find in the examples you reference that the Kings do make the law, albeit with some participation by wider society. In Byzantium this is certainly true, the Corpus Juris Civilis issued by Justinian being just one (and older) example. Emperors periodically issued new laws until the end of the Empire. Even in England the Magna Carta is a very new thing in our period and the arguments whether the King needed Parliament (which of course didn't exist yet) or not to make new law will go on for another 400 years.