I think there's some kind of ventilation shaft you have to jump down into. Like a sewer cover kind of thing.
Help: I am at the end of the furnace level having sliced the rather large chap into human sushi and got in the lift with takes me to the top of the building.
Having killed the two chaps where the lift stops, I cannot find anywhere to go. The door is locked and there do not appear to be any other exits. I have tried 'dropping' off the platform the one below but this always kills me before I have fallen anywhere (i.e. in contrast to the earlier part of the level getting into the waste buckets where you only die when you hit the ground) so guess that this is not the solution.
Thanks for the help.
Any ideas of where I should be looking and what it looks like?
theres a big pipe that you climb on, and then there is a whole in it at one end, with some sort of hatch that is open. if my memory serves me wrong.