What led Steve Jurgenson to follow Chip, Stevie and Susan Carol, according to his story in Chapter 18?

Steve Jurgenson says he was following the kids yesterday to find out what Chip was mixed up in. Jurgenson says that Coach Graber called Jurgenson to check on Chip after the security guard went to the coach the night Stevie and Susan Carol first got into Chip's room by saying Stevie was Chip's cousin.