It should really go somewhere outside. If the hob is on an outside wall, then the hood can discharge very easily via a pipe through the wall, otherwise it would need a longer pipe with a more circuitous route. The discharge (mainly steam) can be via a duct up a few inches and then back through the outside wall, or up through the ceiling and then to an outside wall somewhere, or around the kitchen just below the ceiling to an outside wall. It all depends on the circumstances.