The Perimeter Note is one of Bill's reminders, resting on top of a drawer in a back room of the Marquis Music Store on Lincoln's main street during The Woods section.
Bill is annoyed that infected are turning up too close to the church safehouse and decides it is time for another sweep of the weak points in his perimeter.
It hints at how Bill lives: alone in a church, patrolling and patching his defenses. You reach that church yourself later in the chapter.
Bill's Map
Bill's Map is Bill's hand-annotated map of Lincoln, on a table with an engine near the door you come in through at his safehouse, right after you meet him.
Bombs Note
The Bombs Note is on a table to the left in the side room off the nave of Bill's church. Go into the room on the right, the one Bill makes clear is not the confessional.
Boy's Diary
The Boy's Diary is on the windowsill of a child's bedroom painted with stripes and cars, upstairs in the second house with the dry pool in the new territory beyond the church graveyard.
Fences Note
The Fences Note is another of Bill's reminders, on the table in the back room of his safehouse near the pipe he handcuffs Ellie to.