It is likely a pathing bug as stated. However to get a hint of what the button is form read a letter on the desk on the floor with the button and if you have explored all of the underdark in the EA you may recognize the name.
If you have a shovel in your inventory you can dig up the grave near the sussur tree and get the collar which is tied to the button. It's a food despenser for the owner's dog, which is why it was so low to the ground.
