The Kitchen note is an artifact in the Left Behind DLC flashback chapter Fun and Games, set in a Boston mall. Ellie finds it on the counter in the kitchen of the mall's restaurant while she and Riley explore.
Winston calls himself a jerk because he came back early from patrol, found young Ellie crying beside his tent while petting his horse, and slipped away rather than talk to her. He notes she seems cheerful and full of questions but is hiding a lot of sadness.
It is a rare outside view of Ellie at the military school before Riley comes back into her life. Winston also wrote the Warning note found earlier in Mallrats.
No Pun Intended: Volume Too
No Pun Intended: Volume Too is Ellie's second Will Livingston pun book. In the main game it is an automatic artifact in her backpack at the start of Lakeside Resort. In Left Behind, Riley gives it to her during the flashback chapter Fun and Games.
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.