Sauron is meant to be the big, scary Dark Lord. Yet he captures a Dwarf who has one of his rings of power, and doesn't even bother to strip him of everything he has, slay him, or keep a watch on him. That's dodgy management; that's poor decision-making if you ask me.
Related point: could Sauron not sense that Gandalf was in Dol Guldur? I'm presuming Sauron was there at all times. If Sauron was hiding behind the curtains in Rivendell I'm sure Gandalf or Elrond would have felt something.
