That small pathway escaped me after checking every outdoor tile. Probably because of the orientation of the building, most Rpgmaker games tend to put obvious elements on bottom-facing building walls where they are easier to display.
Solved.
The leader of the Deadhanders gave the mission to get inside Monolito and open the loading doors for them. I have found the elevator, the key card and the control room, and I have restored power. But I can't find anywhere a way to open the loading doors.
I have been at the loading doors in Monolito Exterior, but they don't budge, nor is there any switch nearby.
From their positioning on the exterior map the loading doors must be NE of the entrance, but the logical way is blocked by boxes.