What clue do the boys discover in margo window?

In Paper Towns, Quentin takes a look out his bedroom window and realizes that there is a poster which can only be seen when Margo's window shutter is rolled down. His friend, Radar, identifies it as a poster of the folk singer Woody Guthrie. After this discovery, they are sure that Margo has left more clues to reveal, so they ask Margo's sister if they could search her room.