Near the beginning Colter is driving his truck and pulling his Airstream through downtown Camden. Scene switches to Colter exiting his truck looking for the Finn and Shell. However, his Airstream is no longer being pulled by his truck.
When Colter arrives at the girls house and sees the door broken, he removes his handgun and the hammer is already cocked. A few seconds later, you hear the hammer cocking even though the scene before, it was already cocked.
Except it's common to carry single action automatics like the one shown 'cocked and locked'. Meaning the sound was Coulter taking off the safety. The only thing slightly unusual was that he steadied the gun with his off hand while doing it.
A building in the beginning is marked "US Post" and what appears to be USPS logos which implies it is a Post Office. However, a United States Post Office would not be marked in such a way with only "Post".