The Warning note is an artifact in the Left Behind DLC flashback chapter Mallrats, set in Boston. It sits on a table in the room next to the circuit breakers Riley has Ellie flip to bring the mall's power back.
Winston asks for more patrols for the third time in three weeks. With the Hartford quarantine zone collapsing, refugees are pouring through the poorly guarded mall, many of them already infected. Midway through his complaint he gives up, curses, and admits nobody will read it.
The same soldier wrote the Kitchen note later in the mall, which shows he knew Ellie from the military school.
Wanted poster
The Wanted poster is an artifact in the Left Behind DLC flashback chapter Mallrats, set in Boston. It lies on a table in an apartment Ellie and Riley pass through on their way to the mall.
Boston Q.Z. Map
The Boston Q.Z. Map is the first artifact in the game and you cannot miss it: it is already inside Joel's backpack when you pick the bag up off a table in the smuggling tunnel during Beyond the Wall.
Davidson's Firefly Pendant
Davidson's Firefly pendant lies underwater in a stone rotunda in The Capitol Building section of The Outskirts. It belonged to Melinda Davidson and is easy to swim past.
Docks Note
The Docks Note is a gripe from one of Robert's smugglers, lying in the big warehouse at Mission Hill Pier where a fishing boat hangs over the loading bay. Look on the right side, against the railing behind a blue trash bin.