What happened to Mr. Pickett in Turtles All The Way Down?

Much of the novel’s plot involves the search for Davis Pickett Sr., a billionaire who has gone missing. Ava and Daisy get involved in the search because Daisy wants to claim the reward. The plot gets more complicated, however, when his son, Davis Jr., provides the equivalent amount of the reward money to dissuade the girls from searching for his father.
Towards the end, Mr. Pickett’s body is found in a large sewage pipe or tunnel. The girls notice there is a strong odor, which leads to the discovery. The author leaves the exact cause of Pickett’s death ambiguous, however; he offers an article saying the cause was “exposure.” A bizarre clause in the will stipulated two different legacies depending on whether the body was found or not, one with the money going to the children and the other having it go to a lizard. This leads to a decision by Davis Jr. about revealing the body’s location despite the ensuing financial loss.