There’s a uncollidable floor that you can fall through outside the main door on the facility, right next to the mailbox under the angled wall piece, you can fall through it and get under the map.

Oh, yeah, that's intentional. Not only as a secre, but also in case furniture clips through the floor - there are a couple events that may cause that to happen, and that "crawlspace" exists so you can get your desk/chair/couch back in case that happens.