The Note to Bob is caught in the barbed wire on the ground at the end of Lincoln's main street, past the camper van and the Marquis Music Store, in The Woods section. It gives you the code to a safe.
The writer refuses to leave town without the safe and tells Bob to help Brad load it into the pickup, adding the combination in case they need to open it.
The safe it unlocks holds supplies, so the note is worth the detour before you head deeper into town.
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.