Where can the Ladder card be placed in Saboteur 2?
This was answered by the Designer, Frederic Moyersoen on BoardGameGeek:


  Any card you place in the gallery must match with an existing corridor. There is no exception for placing a card with a ladder.


And here:


  As the ladder is always connected to the start card, you can place this card when the connection is interrupted.


Taken together, the ladder card must be placed so that it connects to a path.  That path need not lead back to the original ladder card.