Aperture Science- The PCs are attached to a research team (possibly from the Fraternity of Order, depending on when in the timeline the adventure is set) investigating the timing and details of spontaneous portal formation in Sigil, ie. how long does it take for a new bounded space to become a portal and is there any pattern to where they lead. Of course occasionally belligerants come through the new portals, and also sometimes the party is sent through the portals to scout the other side.