Maybe it's been like this in the game and I didnt realize it, but I just had 2 survivors escape through the hatch.
Is that possible?
It's possible with a key.
The hatch spawns when the number of repaired gens is one more than the survivors still alive, so if they repaired 3 gens and there were 2 survivors alive, then the hatch spawned and they opened it with a key.
Next time kill the one with the key first and make sure noone else picks it up
Oh i didn't realise that's how hatch spawning works. I remember me and some other survivor spent ages looking for the hatch with my key but we only did 2 gens