Useful questions that in-universe characters could ask:
What kills the people sent out to clean?
Why, despite us clearly having been down here a very long time, does it seem that everyone sent out to clean dies after about the same amount of time? Why is the threat not diminishing over time?
Why were these very well stocked silos ready and waiting and apparently very well matched to the threat outside? (In the books, Lukas does ask this question and gets an answer, and the answer is correct but incomplete).