There is a building at the start of the map where you get your first alternate route. Climb the ladder before going through the first barrier into town. The building with the ramp has a closed door without collision so you can enter where it shouldn’t be possible.
I can’t remember where, but there was another house with a wall that didn’t have collision and you could go outside the map.
Then a building right before the open area where 3rd tome can be found. Sienna found a tiny spot to direct the teleport to the 2nd floor. Going down the stairs lets you go through the far wall outside map bounds to the area under the bridge.