This page of spoiler-free The Last of Us 2 guide contains a list of all secrets found during Infiltration. This is the final stage of Seattle, Day 3 chapter and you play as Ellie. Infiltration is not a long or complex mission and as a result there is only a single secret to be found.
Infiltration - collectible list
During Infiltration, there is only one secret to be found:
1 journal entry
Proceed to a section where Ellie is attacked by a dog during Infiltration level.
After dealing with the animal, reach a room with sleeping bags and a table for communal meals.
In this room, you must find an interactive bag that contains a medallion engraved with Owen Moore's name and surname. This discovery will motivate Ellie to make an entry in her journal.
Warning - Do not open a double door with the crab image before you have unlocked the journal entry. Interacting with the door will activate a cut-scene, after which you won't be able to return to the previous area.