I'm looking for some insight and alternative angles over a murder investigation.
In our saga, house Bjornaer has established a form of limited membership for ungifted people, similar to that of Redcaps or Lartae. One such person is suspected to have been murdered using magic, and a quaesitor is asked to help investigate.
The victim is elderly, and they died of apoplexy. Even though this does not seem unusual for an old person, the players, for political reasons, checked the corpse for traces. They found unidentified spell traces, and that's when they called in the quaesitor.
Upon investigating, the traces are found to be a Rego Vim tunnel effect, and a Perdo Corpus effect related to aging and disease. Both spells had their sigils erased by Shroud Magic (or something equivalent).
Consulting the ghost turns up no useful information as the target was hit with the spell via a tunnel, in their sleep.
I have not played a Quaesitore before, but I imagine they would follow three main paths of inquiry:
Mundane questioning to see if the victim had enemies, or involved in other plot elements that could invite death. Include close examination of all items belonging to victim.
an intangible tunnel implies an Arcane connection, so attempt to determine how that was acquired. Examine the body for missing bits (eg a lock of hair snipped out, recently lost teeth, etc). Ask about victim's habits and deduce where AC could be taken. Also check things like comb that may have hair - who had access to comb, etc.
Examine the magical traces for the slightest clue. Qs are supposed to be good with magical traces (part of their Hermetic Prestige). This particular Q may not be so great, but would the suspect know this? Check for false leads.
Remember, a Rego Vim tunnel really narrows down the suspect list.
Asking around would turn up that a number of people within house Bjornaer had a reason to hate him. Problem is, it's not one or two, it's a bunch.
There is no apparent damage to the body (apart from apoplexy), no conspicuous lock of hair is missing, etc. Members of his household have not seen unknown people around recently.
Use HoH:TL p.70ff Quaesitorial Magic, especially p.71 Acute Sense:
With this ability an altered, hidden or even a destroyed magical trace, may be sensed.
What to roll then follows in the text.
Look also up p.71f Shroud Magic. In the end:
The use of Shroud Magic can be known by the presence of an accompanying Muto Vim trace or by the Acute Sense ability. Unfortunately the trace of the Shroud Magic also shows the false sigil. Therefore in well-planned crimes, spell trace detection can discover what spells were used, but not the sigil of the casters.
So a Quaesitor might from then on try mundane investigation to determine likely suspects, check their motives and alibis. If she should come up with convincing suspects like the bunch of Bjornaers you mention, she might next Tribunal ask to have them testify and make sure, that the Tribunal expects a Quaesitor to do (p.64ff) Endorsing Testimony on them.
Don't rely only on magic. Regular investigative skills and process are also valid:
list the enemies, their motives (minutes of past tribunals can be quite useful to identify opponent to the change he brought)
cross-reference with the rumored mages' expertise: who is skilled in Perdo, Corpus, Rego, Vim
possibly contact Mercere's house to see if in the past few years certain labtext have been requested, and by whom.
If nobody seems to fit the bill, could it be a conspiracy of two (or more) mages ? The more people are involve, the more difficult it is to keep a secret: discreet meetings, exchange of messages, etc.
Gathering AC can be done by grogs. Who are the latest grogs who have been recruited ? What is their background (this part is perfect investigation for companion or grogs, who will look less threatening or intimidating than mages).
What was the reputation of the deceased amongst grogs ? What he popular, ignored or hated (it will help understand the means of gathering the AC).
Also, as you design the investigation, make sure not to make it bullet proof: if the murderer is designed to be an expert assassin, there is honestly little chance to ever find him out. On the other hand, if it is a regular mage (with center of expertise no linked to murdering people), this one would had to build up some skills (spells, and possibly Arts) and possibly deal with some personal weakness. This one should lead some bread crumbs, because he might had to use unusual (for him) large amount of vis to bypass the Aegis, or to shore some lower arts weakness - gathering two rooks of virtus for example might no go unnoticed.
You make very good points, thanks! In this case, both the perpetrator and the investigator are NPCs under my control as SG, I just wanted to make sure I don't get locked into a loop of my own logic, and that case gets a realistic approach. These are pretty good ideas for avenues a quaesitor should pursue.
Finally, don't forget how the Order justice's work: it is as important to have evidence than to gather enough support to convict the mage. So information gathered might not be considered as proof as defined by modern procedural investigation, but if the PCs gathered enough circumstancial evidences, combined with political backing and a grog confessing of some dodgy AC gathering, it might be enough to get the culprit marched.