Thanks to Nita's tendency to follow internet fandoms like her totem, I also know the empty set symbol as a sigil for "operator", which due to a certain web series is also largely regarded as Infrastructure for Slender Man. Who is entirely fictional (believe me, I'd know otherwise; that creature is a classic loyalist, especially of a Destroyer or Psychopomp, depends on design for intimidation or fine manipulators/speed), but I heard from Cichol that certain mages are attempting to create and summon a goetia of it as a personal assassin and tool of intimidation. [Because that's not at all how he took over the world in a story, nooo.] So it's entirely possible you're now host to some of those same Seers and your friend is being targeted for a prototype test. [Though thankfully, one he is not scared of. He got really bored of Marble Hornets soon as he figured out that Source business. By the end he was screaming at Jay to get a grimoire, or at least a clue.]
Even if it's not, I'll look into it. Mathematics symbols as graffiti in general are things that generally have the Temple's mark in general.
(OOC: Given certain events related to Slender Man in the real world, I think I may want to post a trigger warning for the future in case it is discussed. I am avoiding it out of good taste, but if it is brought up...)