I first detected the trap on which the cursor is located the second time i entered the room (didn't go farther than the door space, i think i waited there a bit for garrison activation to end).
Later, while waiting for the investigation squad to leave after i explored the top path and jammed the garrison, i noticed that specific trap didn't appear on the map.
Upon entering the room, i could see it again without having to re-discover it, and it stayed there afterwards.
I don't think i have any other information that could be relevant (i did use a layout(zone) hack on the terminal above, but it didn't reach that far, as can be seen from the green tiles above the garrison).