If you're utilizing a func_brush entity and you have a leak in your map that's shooting into the skybox, first thing I suggest you go check is any func_brushes you have in your level. Func_brushes don't act like func_details in which when you search for the pointfile leak and it comes up with the path. No, the func_brushes act like normal brushes. So if you have a leak in the map; Hammer won't know where to put the leak and it'll just go into the skybox. (I'm sure there's a reason for this, origins maybe? Idk.) Point is, this is something that should be looked out for if you can't find a reason for something is leaking.