a matter of Hermetic Law - and of Calebais

I am very gratefull for the idea, and they certainly will (as soon as I get a fix on those tribunal acknowledgements discussed elsewhere)! I plan to have them announce this in a glorious way to the gathered Tribunal at the very same day as they earlier swore the PCs to silence! What an affront! What a fantastic provocation! Ahhh the glee!

And as a sidenote, the fate of Calebais and how it tore it self appart from within - even if it wasn't a Diedne assault that killed them aa claimed - is such a nice touch of underlining the theme of strife and internal conflict between magi. In that sense, even if unrelated to the Schism War, Calebais (and now Calebais Renata) is a model analogue picture of the Schism War on an atomic scale.

Presumably your plot demands the Quaesitors threaten the PCs with trial instead of simple Wizard's War. In that case ... the PCs weren't foolish enough to use any magic when prying into Ornath's clearly advertised private business, were they? A tribunal probably wouldn't consider this a capital offence, but maybe it will if enough pressure is applied.

It would probably be necessary to add other, spurious charges. A number of senior figures can testify that for various reasons invading the ruins endangered the Order. Perhaps it's been mentioned that a notorious demon invites ambitious mortals to summon him in the ruins of Calebais. The magi should have been informed, and if they deny any foreknowledge, clearly they PeMe'd themselves to erase their guilty memories. If you want the Guernici to be even more unfair, a sufficiently powerful master of Mentem could plant false memories in the PCs' minds (duration Moon, probably) of having committed serious crimes. Or they could threaten the PCs with a subtly rigged version of Frosty Breath of the Spoken Lie when they try to testify in their defence.

Getting back to the original question, I don't think Quaesitors have a right to proscribe certain areas (even their own sancta). However, if a competent senior maga, or preferably several, concludes that interfering with an area is likely to endanger the Order, and has a Quaesitor mark it as such, then anyone who trespasses can be assumed to be reckless at best, and is entirely responsible for any accidental harm they cause.

Yes. Or at least threaten them without ever making it to a trial...

A bit tricky - since Ornath died a long time ago. And he finished up in the ruins long before that.

Or if the magi at least might be led to believe this to be the case.

I've been considering that as an option. I guess I'll wing it when we get to the scene, as the Quesitors involved don't want to push to hard either, as they might fear it to provoke or push the PCs into a corner from which they might do something desperate... and foolish.

A very good idea with a nice touch.

Might be a last ditch effort of the quaesitor in charge, but not his first avenue of handling it, but...

in the history of the order...

1011 1149 Stonehenge: The now Nameless Covenant, in Anglesey, is destroyed in one of the last major conflicts of the Schism War. Quaesitores investigating the covenant’s remains find something or some things in the ruins that lead them to obliterate the site entirely and destroy most of its records. [HtM]

Looks like you have a similar situation but they got a bit sloppy in the cover up

There truely is. And it is a thing inspiring me for the present situation, whereas the Nameless Covenant is something reserved as a treat later in the saga.

The difference in how the two places might have been that the Anglesey one was a genuine Diedne covenant and it was destroyed in the last days of the War, where as Lumen (our intepretation of Calebais) happened prior to the War and actually wasn't related to it (though claimed to be). Lumen lay undistrubed and perhaps put aside when the War ended and all were weary from the conflict.