Below is a list of hidden places in Multiplayer and Single Player.
Upheaval (The house next to the burning house):
Stand on a windowsill and face the side and you will fall onto a ledge below.
Upheaval (The burning house):
Go up the steps and out the window in the same way to hit a ledge below.
Courtyard (At the end of the courtyard):
Go into the shack and turn right then use the jutting out portion of wall to hop into the tree.
Hanger (The porch outside the hanger entrance)
Near the hanger there is a large white carved stone which if you jump on and try and hop over the fence you will instead land on it.
Added by: Sanzano Jul 16th 2010, ID#6032