The dev got lazy and forgot to put a wall in on the black garden mission. To find it start the mission and once you get past the gate look to your right and behind the dead ghost there are some ledges you can jump on once you get on the ledges your notice a hole in the wall just walk into the hole and just like that your out of the map. Also in the lighthouse there a hidden area off to the left of the spawn that you can get into To find this spawn into the lighthouse head to your left. As you get to the edge look down and to your right your see some ledges you can jump on follow them and you get into a hidden area