There are a few clues that they find in Turtles All the Way Down that help them find Mr. Pickett.
In chapter 3, Daisy and Aza go across the river by Pickett’s golf course searching for him. The camera in the woods by the river is not password protected so that they can download the pictures to their phone. In one of the photos, they see Mr. Pickett in his nightshirt going toward the river to escape.
I swiped through more pictures until I got back to September 9th, and there, yes, in the shade of green I could see the back of a stocky man wearing a striped night-shirt. (Chapter 3)
The picture of Mr. Pickett helped them understand how he had escaped, and that gave them a good idea of the direction he went.
The Notes from Pickett’s Phone
The notes that Noah gives to Aza because he wants to find out what happened to his father are another clue. Davis doesn’t want Aza to keep up the search, but Noah, his younger brother, asks her to keep looking and gives her the notes he got off of his dad's phone. Of the notes, one includes the phrase “The Jogger’s Mouth” which is essential because it is eventually what helps Aza and Daisy realize that the jogger's mouth is the entrance to the tunnel at Pogue’s Run. She tells Davis, and he calls in an anonymous tip to the police who find his father’s body in the tunnel system.