Filtering the storyforms in the Analysis section, it appears to me that Situation and Psychology have the most mysteries. There are one or two in Activity and none in Fixed Attitude. Of course, this isn't a very big sample size, I might be missing some due to my own lack of familiarity with all the stories, and "mystery" is admittedly a fairly nebulous label.
I think determining whether a mystery falls into Situation or Activity has a lot to do with with whether the crimes are ongoing as @jhay alluded to and whether they pre-exist the story. In The Silence of the Lambs, Buffalo Bill kidnaps the senator's daughter, which is an inequitable Situation itself on top of Buffalo Bill being out there and needing to be caught.
I was just looking through OS Activity storyforms and came across Scream, which has a mystery at its core and a series of crimes happening throughout the story. The status quo has been shattered by a series of killings, and they must be stopped. Whereas in_The Silence of the Lambs_, the status quo itself is problematic; a serial killer operates in this world with impunity.
What is Dramatica in 30 Seconds? It sounds interesting, but I've never heard of it.
The goal for this story would have to be "stop the thief." This is the second story in a series, and I think I confused myself thinking about a broader storyform that covers a few installments. From an objective perspective, the school has been targeted by a shady character because of all these students with these magical devices (but that's in the big picture), and the thefts play into that (which is what this installment is focused on).
When you start thinking about the source of inequity, sometimes you ask yourself weird questions. The school has been infiltrated by a person with ulterior motives.. is that actually inequitable in and of itself? This feels more like a Situation to me and for a couple of days my answer to this question would have been "Yes, of course," but right now I'm thinking, "No. It doesn't matter what the person wants to do. Simply being in the school and going to class isn't inequitable (assuming the story isn't about the student who didn't get to attend because her spot was taken by the thief); rather, the inequity arises when the thief acts. Hopefully, that's correct.