Cyberarmy
Love fool
What are the thieves, I wonder? I always saw "The thief has escaped," pop up when one of the floating skulls would get away, but their circumstances make be think they're some other enemy type that I never even caught.
They are just standart wraiths that give some money upon death. They always come back when you reset the bell. You can use stealth tactics to kill them easly, they won't run away if they can't see/feel you.