Running through campaign I found a glitch where if you break off a particular turret it will cause you to fall through the map. At which point you can explore nearly the entire map layout from underneath the intended layer.
I have a short video of it here -
https://www.halowaypoint.com/en-us/games/halo-5-guardians/xbox-one/game-clips?page=1&gamertag=xII%20Kingz%20IIx#ugc_halo-5-guardians_xbox-one_gameclip_xII%20Kingz%20IIx_83e394ac-be63-4141-b2ff-9768d123670d
Also me running around after I was outside the map -
https://www.halowaypoint.com/en-us/games/halo-5-guardians/xbox-one/game-clips?page=1&gamertag=xII%20Kingz%20IIx#ugc_halo-5-guardians_xbox-one_gameclip_xII%20Kingz%20IIx_7172bf54-0b87-4064-b5ea-fa6cf63fcd7f
Then I have found a spot in Mission 9 where you can fall behind some of the buildings/set pieces and get stuck without any way to out.