I was playing the map with r_drawrenderboxes 1, and found a secret I don't think anyones found before.
If you go to the corner with the grass field with r_drawrenderboxes on, theres a render box in the very corner. If you press E on the box, it teleports you inside a building that's in the very corner of the section of the map with all of the warehouses.

Have you guys ever seen this before? It's not in the original gm_bigcity, just this version. It's completely invisible unless you have r_drawrenderboxes on.