The unknown camera feed, labeled as such inside the Security Camera, is a feed that is completely black except for some instances of static. It is unknown where or what this camera is.
Mongrel is most likely somehow related to the missing camera. Upon entrance to the West Janitor's Closet, Mongrel nigh-immediately jumpscares you by leaping at you from the left. If you manage to turn your head to face him, however, you can see, in the lower right-hand corner of the wall behind him, there are two wires extending from the ceiling into the floor. As the room is very dark, this is hard to see. The larger wire has been torn in two.
That in itself may not be a huge indicator (that could be any wire), until summoning Mongrel using the Security Camera comes into play. When you view the West Janitor's Closet, the majority of the dark room is covered by the silhouette of Mongrel's mask. However, to summon in Mongrel, you have to follow certain steps. The last of these steps are to view the unknown feed before viewing the West Janitor's Closet. Then, Mongrel will be gone, and for a split second before the camera auto-exits for his jumpscare, sparks can be seen near the torn wire. This indicates that it's the wire for the camera, and the reason the camera is black is because something in the West Janitor's Closet ripped through the wire. This is most likely Mongrel, as Mongrel aggressively kills anything or anyone else that it sees inside of its perceived "territory", not allowing others to get anywhere near the wires.