I finally bought Betrayal in Baldur's Gate and gathered with some friends to play it. Before the haunt, we stumbled with a couple of event cards that seemed to suggest that a Monster should be placed on the board. But we weren't sure.
For example some cards, such as the Shambling Form, clearly have a resolution for the "monster" (it explodes).
But some other aren't as clear. The Zhentarim Opportunist is one such case. The event clearly mentions an opponent/monster but we don't know what happens with that monster.
Are all events one-shot actions (unless specified otherwise)?
I'm inclined to believe that the text in vent cards is mostly fluff/flavor. And the fact that a "monster" is indicated is just for ambience. That way, the Zhentarim is understood to have left/fled right after the event is revolved (regardless of the result from the attack).