They pretty much said you have to follow the pattern of the NPCs to get erased (unless you mean that is getting caught). Main char guy (his name escapes me) got caught right when he woke up and walked up to her. And I got the impression that Yuri spoke with Angel a few times from the discussion on the rooftop, so she probably knows a lot more than she's telling him up front.
Oh, and something that bugs me about that discussion: How long has Class 3S (I think that's what they called themselves) been fighting that Yuri knows that the teachers, students, and dead people don't age in that world? I can't help that feel he should have come right out with that question, but I can forgive him given the circumstances. Maybe I'm reading too far into it and trying to find plot points where there aren't any.
