The Angry Letter in Silent Hill: Downpour is located on a door on the third floor of the Hillside Apartments. It is from an angry resident to an apparent thief giving him a fair but firm warning. Murphy Pendleton can find some stolen goods in the thief's apartment, which leads to the "Stolen Goods" sidequest.
I'm on to you, you junkie asshole. Next time something goes missing from my apartment, you can expect the cops to come knocking on your door. And if it's not them, it'll be me, and my knock is 12 gauges if you catch my drift. So if you don't want an ass full of buckshot, I suggest you stay the fuck away from my apartment. Last warning.