Memory #1 — The Flat
The first Memory becomes collectible the moment B-12 joins you. Before that, you can walk past it and not see a prompt.
Location: the apartment where you free B-12. Look at the wall to the left of the workbench — there is a framed photo at cat-height. Press triangle when prompted.
If the prompt isn't appearing, you haven't completed the B-12 cutscene yet. Trigger that first.
Why it matters: Memories are tied to the I Remember! trophy (collect all 27) and the platinum. They also unlock B-12's backstory between chapters.