If you cross the river using the broken bridge you get to a swamp with one single house. There are some bandits waiting before the house. The door is locked, if you open it you get the message "key needed" which indicates that the door can be opened if you get the correct key.
I have already played through chapter 1 once, but I never found the key to this house. Is it part of some specific quest or is that a dead end that was never implemented in the final game?