That - it's one of the things you can learn to do with the psychic network. Sensing when a Traveler is near you, and where they are. Liminal space is usually small enough that if I position myself right, I have the range to sense everyone who's still around.

[He hopes Matthew doesn't get weird about that. Some people have disliked the thought of Armin being able to detect them that way - disliked it emphatically. But the man has already shown that he appreciates candor, even if the topic isn't pleasant.]