Image: Naughty Dog / PlayStationOfficial screenshot from Naughty Dog / PlayStation.
In Memoriam is the bronze trophy for picking up the crumpled note Bill throws away in Bill's Town. After Frank's letter is found and handed to Bill in the High School Escape section, Bill reads it and tosses it in the garage of Frank's house; grab it off the floor.
Frank's letter tells Bill he is leaving town and spells out how fed up he has become with Bill. It also mentions the battery Bill wanted.
Look around Bill's feet in the garage right after the scene ends, before you move on with Ellie.