The Case of the Tiger on the Toilet — Brian C. Jacobs

Junior-highers Tyler, Scooter, and AJ know the ghost stories about the old Safari House are only stories. But when AJ starts seeing ghostly animals, the boys put on their detective caps to investigate.
Was there a mass breakout from the zoo, or is the house really haunted? Mysterious clues point them to a teen from the local high school, but how is he involved?
And when the Enigma Squad solves the secrets of the Safari House, will they find something more sinister?