In the first suburb, in the left wall, near the big tall tower where the Arachnoid spawns, there's a small alley with some ammo.
As of #1, but instead at the end of the wall is a tree. Blow it up or cut it down to find some items in an area behind it.
In the second suburb, just like #1, there is another secret alley near the button that you have to press to open the sewers on the left side of the Suburb.
Still in the second suburb, go to the rightmost corner of the suburb and pick up an item. This will cause a wall to rise up, and you'll be trapped in a small area, and 3 Minors will appear left, right and behind you (one after you kill the other), and then a Major on the building in front of you.
Near the door, on the left, there is a building with a health bottle behind it. Pick it up to register the secret.
Go up to the door in the end of the second suburb, turn right and then turn left, where you can see a wall. Walk up to the wall and turn left, you'll see a +1 armor shard. Pick it up to trigger a "yodel" secret (spawns some big enemies).
when you're about to jump into the sewer, you'll see a small ledge sticking out of a wall in the pit. Jump on the ledge (best done when facing opposite of the ledge, jumping from the locked door's direction) to find a secret room.