"He'll have to catch us, first," Henley tells him, passing the marker over. A pause follows, and then she asks, "How long do you think it'll take him to notice?"

By 'catch' she meant that she and Jack might be able to outrun Dylan, however briefly, but now that she thinks about it, it might take him some time to notice they did anything. Particularly if they put the pictures back where they found them when they're done -- and for as little time as Dylan actually spends here, anymore, anyway.