Rooftops:out of level glitch
This works on multiplayer and online
If you walk past the subway and face the big door and look to your left there is a bush and if you croutch and go into the bush you can get behind the patch and walk through the door into a hidden room and a big place outside of the level.
Note: it may take a while cause it is a suicide glitch it will usuallly kill you but you can get in.