Must a stench pipe always terminate outside the roof void?

I have bought a new flat. The styench pipe terminates in the roof void and seems to be capped with some sort of whitish (polystyrene?) filter or cap. There is a short length of pipe protruding from the roof apex. Although this is not in line with the stench pipe, it does look awfully like these two should be connected?? Is the cap or filter an alternative? Would it do any harm if I connect the two so that the pipe vents to the ouside? There is a slight smell in the attic but nothing in the flat