Write about: the pattern.
I've got a pretty solid headache, so I think I'll just get to the writing and then go to bed.
"Do you recognize it?"
"I'm... I'm not sure."
"What? What do you mean, you're not sure?"
"I'm pretty sure there's not much room for interpretation on that one. I'm not sure means I'm not fu-"
"Okay, okay. So what do we do?"
"There's no way we open the hatch if we're not absolutely sure it's him."
"But what if it is him? What if he's just standing out there, in the open, with the Hunters closing in on him?"
"What do you want me to do? Throw open the only obstacle between our safety and certain death because maybe that was the right knock code? What if that's a Hunter out there? What if it's twenty?"
"He should have returned by now. It must be him."
"If it is him, he'll repeat the pattern. That's the understanding. You knock. If no one opens the door, you wait two minutes and do it again. We just have to wait."
"What if waiting means he dies right outside our door?"
"Better him than all of us."
"... if the knock comes again, you better listen real hard."
"Oh yeah? And why is that?"
"Because if you're not sure again and it sounds even remotely right to me... I'm opening the hatch."