A castle represents a significant investment of time and power, demonstrating a noble's intent to command an area. You can be certain the response to one's destruction is going to be proportionate, so long as the noble can afford to respond. If he knows anything about the Order, he's going to start hunting. If he has hedge wizards in his reach and knows about them, he's going to start squeezing. I can see a noble asking a local bishop for guidance, and getting pointers to the Order. But the Order's going to move faster, because he's got to ride out, verify the story, start dispatching people to work for him...wizards get to bamf or fly and that's faster.
Setting aside the craft magic option, which is pretty clever, and looking at a standard Hermetic approach...
If the culprit creates arrows enchanted with the equivalent of End of the Mighty Castle, he's going to leave his sigil and they'll be Arcane Connections back to himself. But, if he forgets that, or figures no one will notice, let's consider the Quaesitor's investigation. I would have him ask for a neutral assistant from within the Tribunal, who's reputation makes them trustworthy, but preferably someone fairly skilled in Intellego who can verify his findings. This magus is just here to observe the Quaesitor.
He can question mundanes-- and while they might not be valid for evidence at tribunal, their story lays ground work for further investigation. What happened? Did they see the castle fall? How did it happen? Was it sudden or was there siegecraft involved?
He absolutely must question the rubble of the structure-- using InTe, to learn what the structure itself can tell about the destruction. Where did it first fall apart? Did anything strike it just before it fell apart?
He must investigate the scene of destruction and search for remnant magical effects and sigils.
So what do you need to do this?
Talk to the rocks-- InTe 30 and a lot of concentration.
Talk to the people-- ReMe/InMe/interrogatio of your choice.
Check for a sigil over the ruins-- InVi
Investigate the rubble for a possible arrow? That takes another InTe, either to ask the rocks in the area where the effect hit, or to simply look through the rubble for an arrow-- maybe with a He prerequisite. If he's lucky enough to find the parts, he's got gold. (Remember it's a PeHe30 effect to destroy the parts at AC range [Base 4 + 4 for AC + 2 for Group]) If he just gets the sigil, then he's going to be hunting through the Tribunal for the particular sigil, interviewing members, trying to trace it to the magus or pater who trained the magus, maybe even a filius who can identify the sigil. That can become a wider search unless the sigil is well known by the magus' previous involvement in Tribunal affairs. Let's be honest, really, it depends on how the SG is running the game with this point. If the SG favors a sandbox approach, then this may or may not work based on the arts of the culprit. If the SG favors a more linear adventure progression, then he'll be investigating until he hits the proper point in the storyline to engage the culprit-- but that depends on the culprit's purpose to the story. Is it meta? To an extent, but it helps determine if you're going to be following breadcrumbs or prying the clues out by the skill of your dagger-sharp wit.
Between the previous two effects, he might learn that an arrow struck the castle, and where, and with it (or parts of it) in hand, he can then perform the InCo scrying which would reveal the culprit-- but only once he's checked with the Tribunal's Praeco to be clear on the local rules regarding scrying and received permission. It's possible that the item was stolen, that it was a commission, that the builder was unaware of its use, and so this step is necessary to show the probable culprit's face, but to have witnesses who see that he's just looking long enough to identify the individual to the observers.
Now comes a delicate choice. Based on what (who?) he's discovered, the quaesitor has to decide if he shows up with a pack of hoplites and asks the suspected culprit to come in nicely or goes with a more aggressive stance. The culprit's response will likely dictate the end result, and you could really see the suspected magus responding badly, because he probably isn't going to be happy with the idea. Why would the suspected culprit be guaranteed his Parma? How long would he be held? Would there be an emergency tribunal? All of it could spiral very quickly out of control, and the quaesitor may need to act as a guarantor of the culprit's safety to maintain his own reputation.
Overall, it seems like a very cool adventure-- whether it be in the box or on the rails-- for a quaesitor.
-Ben.
EDIT: Added portion on sigil investigation.