If it's an ambush, why are you teleporting people out instead of having the ambushing mobs appear?
Regardless, have you tried using a room trigger that prevents people from leaving or entering the next room? You can look at the guildguard triggers to get some ideas. If you think the timing of that looks a bit odd, like a computer freeze where you're expecting something that doesn't happen right away, use an echo. Maybe something like: A slight noise catches your attention and you pause to look around.
