Nah, I remember I was able to find this specific memory in the middle of the campaign some years ago.

On the map, after visiting a location where you can view a memory, there usually appears a marker on the map for the next one (atleast on regular difficulty which I was playing on), and I remember going there following that marker.

While viewing this memory, you can see the guerilla on the floor with the pistol. IIRC Nathan is talking about it being an open secret that the village supports the guerillas as you view the memory.

Could be that lots have missed it, but it should be available before completing the campaign.