The postcard.
This is a postcard received by Jeff directly prior to the events of Cops Checked, No Body. It bears the faded image of a road and the words "Greetings, from the land of ashen waste". The word HABIT is written in the lower left corner and some people interpreted it as a "signature" of sorts.
Following the postcard's clue, the crew went to what has been identified as the old Lambertville High School, and the events of Cops Checked, No Body followed. Convinced the post card was supposed to lead elsewhere, the crew took suggestions from players, culminating in Ashen Waste and Centralia.